The Military Intervention Project: A New Dataset on US Military Interventions, 1776–2019
Sidita Kushi and Monica Duffy Toft. Original post here: https://journals.sagepub.com/doi/epub/10.1177/00220027221117546
Full article here:
While scholars have made many claims about US military interventions, they have not come to a consensus on main trends and consequences. This article introduces a new, comprehensive dataset of all US military interventions since the country’s founding, alongside over 200 variables that allow scholars to evaluate theoretical propositions on drivers and outcomes of intervention. It compares the new Military Intervention Project (MIP) dataset to the current leading dataset, the Militarized Interstate Disputes (MID). In sum, MIP doubles the universe of cases, integrates a range of military intervention definitions and sources, expands the timeline of analysis, and offers more transparency of sourcing through historically-documented case narratives of every US military intervention included in the dataset. According to MIP, the US has undertaken almost 400 military interventions since 1776, with half of these operations undertaken between 1950 and 2019. Over 25% of them have occurred in the post-Cold War period.
military intervention, dataset, foreign policy, war, United States
1Bridgewater State University, Bridgewater, MA, USA
2Fletcher School, Tufts University University, Medford, MA, USA
Sidita Kushi, Department of Political Science, Bridgewater State University, 131 Summer St, Bridgewater, MA 02324, USA. Email: firstname.lastname@example.org
Foreign military interventions are now routine endeavors in international relations, especially in response to intrastate conflicts (Pickering and Mitchell 2017). The United States (US) has readily enforced this kinetic trend. According to our research, the US has undertaken almost 400 military interventions since the country’s founding in 1776. What is more, these interventions have only increased and intensified in recent years, with the US militarily intervening over 200 times after World War II and over 25% of all US military interventions occurring during the post-Cold War era.1
Some scholars have explained such increasing interventionist trends as part of the new norm of “contingent sovereignty,” which explicitly challenges the traditional principle of non-intervention in the internal affairs of other states (Ramos 2013, 143). Particularly regarding the US, one perspective is that the country is evolving past its Cold War doctrine of containment toward acting on norms related to humanitarian intervention (Finnemore 2003; Haass 1994, 14). Indeed, military interventions in Bosnia, Kosovo, Libya, and Somalia all held some humanitarian justifications, but these interventions have typically failed to achieve their humanitarian and democratizing objectives (Pickering and Kisangani 2006; Walker and Pearson 2007; Gleditsch, Christiansen, and Hegre 2007). Other scholars argue that US military interventions harm foreign citizens and diminish US security goals, weakening interventions’ both humanitarian and interest-based explanations (Peksen 2012; Aslam 2010; Dimant, Krieger, and Meierrieks 2017). Instead of spreading democracy, these interventions tend to transform target states into illiberal democracies — at best (Walker and Pearson 2007; Gleditsch et al. 2007).
Yet accounts of US military interventions to promote geopolitical interests cannot explain the dynamics of the post-Cold War era either. If the US primarily intervenes when its security interests are threatened, we expect the US to intervene less in an era void of peer competitors where fewer vital interests are arguably at stake (Taliaferro 2000; Waltz 1979; Art and Jervis 1973).
The restraint literature further argues that US foreign and security policy since the Cold War has been a hyper-militarized failure, often in opposition to vital US geopolitical interests (Posen 2014; Gholz, Press, and Sapolsky 2017; Mearsheimer and Walt 2016; Layne 2017). Furthermore, some scholars contend that the US uses force abroad without a clear organizing principle, and thus its military missions have had disastrous long-term and unintended consequences (Arreguín-Toft 2001; Aslam 2010). Toft (2018) has labeled current patterns of US military engagement as kinetic diplomacy, diplomacy solely through armed force. Indeed, in the past years, “while US ambassadors are operating in one-third of the world’s countries, US special operators are active in three-fourths”. This raises important empirical questions that require comprehensive data on US military interventionism across history: has the contemporary US increasingly relied on force as a foreign policy “instrument? What do patterns of US military interventions look like across time and place? Do these patterns promote US national interests?
Examining this reliance on force motivates the Military Intervention Project (MIP). MIP is a comprehensive dataset of all US military interventions since the country’s founding. MIP measures potential drivers of intervention and the domestic and international costs of US military involvements, combining over 200 variables that allow scholars to evaluate key theoretical propositions. Compared to the most robust existing datasets of US military intervention, MIP doubles the universe of cases, integrates a range of military intervention definitions and sources, expands the timeline of analysis, and offers more transparency through historically-sourced case narratives of every US military intervention included in the dataset. MIP rests upon meticulous case study analysis of each intervention to verify details across at least three scholarly sources. These case studies include a summary of the intervention, discussion of objectives and outcomes, and a section on any existing data or definitional discrepancies.
This paper proceeds in four parts. We first introduce the fundamentals of the MIP data set. Then, we compare MIP’s list of US military interventions abroad to existing military intervention datasets, focusing particularly on a comparative analysis between MIP and the Militarized Interstate Disputes (MID) dataset, currently the most comprehensive dataset on the topic. This comparative exercise also showcases the MIP case universe and a range of important patterns of US military intervention over time. Our third section applies descriptive statistics, graphics, and means testing to illustrate how scholars can use MIP to assess key theoretical arguments as to why interventions occur, revealing how existing theoretical arguments fall short. The last section provides main conclusions and next steps for the application of the MIP dataset.
The Military Intervention Project Data Set
The MIP measures all instances of US military intervention from 1776 until 2019, alongside key drivers and consequences of these interventions. MIP significantly improves upon existing military intervention datasets by measuring every confirmed instance of the US usage of force, including displays and threats of force and underexplored covert operations, analyzing individual case studies of each military intervention.2 Moreover, it matches these cases to variables related to consequences and causes of intervention, ranging from economic and political to human costs. Relying on a broad historical lens of US military interventions, the project also speaks to long-term trends, dramatic changes, and lasting costs and benefits to domestic and international politics. As a result, MIP can more comprehensively test the explanatory power of international relations theories of intervention as well as the attractiveness of competing US grand strategies, thereby informing critical policy debates concerning American interest.3
MIP contributes at least six innovations to existing resources:
1. Expands the timeline of data from 1776 to 2019;
2. Uses a range of definitions for military intervention, including display and threat of force;
3. Compares definitions and cases across different databases;
4. Measures both short-term and long-term costs of intervention to the US and target states; and
5. Includes covert and special forces operations;
6. Complements the quantitative data with extensively sourced historical case studies of each intervention, providing over 500 new case narratives that confirm intervention across three sources and offers details on intervention objectives and outcomes
Definitions of Military Intervention
A challenging aspect of measuring military interventions is how to define an intervention. Existing datasets on military interventions offer their own definitions, varying from types of interventions to degrees of intervention. To address such definitional issues, MIP has integrated and reviewed cases of US military interventions from these datasets and many other sources, coding each case’s corresponding definition of intervention, as per the list below (definitions are not mutually exclusive).
Military intervention project’s unit of analysis is a US military intervention within a target country, including start and end dates. Consistent with MID’s definition, MIP’s broadest and most general definition of US intervention encompasses united instances of international conflict or potential conflict outside of normal peacetime activities in which the purposeful threat, display, or use of military force by official US government channels is explicitly directed toward the government, official representatives, official forces, property, or territory of another state actor (Palmer, D’Orazio, Kenwick, and Lane 2015; Gibler 2018). This definition includes US military interventions within civil wars, if the US used force on the territory of another state actor. MIP also includes a separate variable that accounts for US interventions against non-state actors within a target state’s borders, such as US interventions against terrorist groups. Scaling hostility levels allows scholars to aggregate and disaggregate different types of interventions and even follow the progression from a US threat to any rise in hostility levels thereafter.
The definition of “US military intervention” may fall under any of the following categories, which each correspond to at least one existing dataset:
1. “The movement of regular troops or forces (airborne, seaborne, shelling, etc.) of one country inside another, in the context of some political issue or dispute.” To separate higher intensity interventions from minor skirmishes, this definition excludes paramilitaries, government-backed militias, and other security forces that are not part of the regular uniformed military of a state. Similarly, “events must be purposeful, not accidental.” Inadvertent border crossings are not included in this definition and neither are unintentional confrontations between planes or naval ships. The definition excludes soldiers engaging in exercises in a foreign land, transporting forces across borders, or on foreign bases. Furthermore, the definition categorizes international military interventions by temporal guidelines so that interventions are continuous if repeated acts occur within 6 months of one another (Pearson and Baumann 1993). – International Military Intervention (IMI) dataset;
2. “Instances in which the United States has used its Armed Forces abroad in situations of military conflict or potential conflict or for other than normal peacetime purposes…Covert operations, disaster relief, and routine alliance stationing and training exercises are not included here, nor are the Civil and Revolutionary Wars and the continual use of U.S. military units in the exploration, settlement, and pacification of the western part of the United States” (Salazar Torreon 2017). – Congressional Research Service (CRS);
3. “A political use of military force involving ground troops of either the US Army or Marine Corps in an active attempt to influence the behavior of other nations” (Blechman and Kaplan 1978);
4. “Any deployment of US ground troops on the territory of another country that included at least 100 person-years” (Kavanagh, Frederick, Povlock, Pettyjohn, O’Mahony, Watts, Chandler, Meyers, and Han 2017). – RAND RUGID dataset;
5. “A use of armed force that involves the official deployment of at least 500 regular military personnel (ground, air, or naval) to attain immediate term political objectives through action against a foreign adversary. Routine military movements and operations without a defined target like military training exercises, the routine forward deployment of military troops, non-combatant evacuation operations, and disaster relief should be excluded” (Sullivan and Koch 2009). – Military Interventions by Powerful States (MIPS); and
6. “Militarized interstate disputes are united historical cases of conflict in which the threat, display or use of military force short of war by one member state is explicitly directed towards the government, official representatives, official forces, property, or territory of another state” (Palmer et al. 2015; Gibler 2018). – Correlates of War (COW) Militarized Intrastate Disputes (MID) dataset.
Comparative Perspective on Military Intervention Project
Existing datasets on US military interventions offer many benefits and serve as valuable resources for cross-checking cases and coding variables. We compare the existing datasets to MIP for the sole purpose of illustrating MIP’s contributions and approach, not to discount the importance of these resources. While several datasets offer information on US foreign military involvements, the main actors, and types of forces employed (air, naval, or ground), compared to MIP, each presents limitations or differing scopes and purposes. The biggest challenges lie in (1) dataset time frames, (2) varying definitions of military intervention, (3) lack of detail and documentation of cases, and (4) how different measures were coded. Crucially, none of the alternative datasets contains comprehensive variables on intervention outcomes, consequences, and other explanatory measures for the US and target countries.
Below, we highlight the most relevant sources of existing data on US military interventions. We explain how these existing datasets compare to MIP before embarking upon an empirical comparison between MIP and the premier current dataset on military interventions, MID.
Existing Datasets of US Military Intervention
Currently, the most comprehensive dataset of US military interventions is the Correlates of War (COW) Militarized Interstate Disputes (MID), which we reviewed extensively as part of compiling MIP’s case universe. MID covers the period 1816 to 2010 (Palmer et al. 2015). While certain versions of the dataset include state actor labels, none include details of each listed dispute (at least not until 1993). The coding of the intervention is limited to a numerical entry. Fortunately, COW recently launched an updated dyadic version of the MID datasets, allowing us to obtain some information on listed interventions via the dyadic pairs (Maoz, Johnson, Kaplan, Ogunkoya, and Shreve 2018). Gibler’s (2018) book, International Conflicts, offers sentence-long descriptions of each intervention in the dataset, but without sources or documentation.
MID is a powerful source of information on US military interventions. Nevertheless, it has limitations, many of which have been thoroughly discussed by Fordham and Sarver (2001) and will not be repeated here. We reviewed Fordham and Sarver (2001)’s revised MID listing, but since their study is only updated up until 1995 and does not measure military disputes in the same aggregated way, we still relied on MID-proper to populate our initial list of cases. Some technical limitations of MID are that it has no threshold for the duration of the mission, which biases the sample of interventions. MID also lacks variables on the size of the intervention. Most importantly, however, MID does not offer detailed descriptions of, or sources for, its case universe of interventions. As we reviewed MID, we noticed that over 82 cases of its coded US military interventions were cases of US non-intervention, as we could not find confirmation of a US threat, display, or usage of force. To be clear, the comparison to follow between MID and our new MIP dataset only serves to showcase the new contributions of MIP and how it differs from other datasets. The comparison is not meant to adjudicate the superiority of one dataset over another.
Military intervention project relied on case study analysis to confirm or reject cases of US military interventions founded within existing datasets and other sources, such as the Congressional Research Service’s (CRS) list of “US Armed Forces Abroad.”4 As part of this process, MIP heavily edited the CRS and MID case universe by aggregating, disaggregating, or removing interventions. These alterations are described within the individual case studies. For example, the CRS document lists Cuba 1814 as an instance of a new US military intervention, but this case denotes a pattern of clashes between US ships and pirates that took place all over the Caribbean from 1814 to 1825. However, the U.S. did not deploy naval ships on a concerted mission to stop this activity until 1822; we thus delete this case, while keeping Cuba 1822 and Cuba 1825 in the dataset. Another example is our aggregation of Spain 1818 (MID 1567). We merged this case with Spain 1816 as both cases refer to events of the First Seminole War. Another example from MID is France 1835 (MID 301). This case refers to a situation where the US was demanding payments from France as reparations for past wrongs. In response to French intransigence, President Jackson contemplated a range of options, including imposing trade sanctions and seizing French vessels. At one point, Jackson asked Congress to authorize the use of naval ships in these endeavors, but the Senate refused to do so and eventually France relented before the US took any concrete steps toward retaliation. At most, we find indication that the US undertook “preparations” for potential naval deployment, but this step falls short of the threat, display, or use of force we require to include the case in MIP. We list our aggregations, disaggregations, and removals in the codebook, Section V, with detailed explanations.
A second dataset of military intervention, the International Military Interventions (IMI) data, limits itself to the narrower timeframe 1946–2015. It includes all movement of troops, but with no measure of size and duration. In this way, the data often include border skirmishes, without properly separating these instances from more extensive interventions (Kisangani and Pickering 2008; Pearson and Baumann 1993). Like MID, IMI also lists cases of intervention that cannot be confirmed via its documented sources. Despite its temporal and definitional limitations, however, IMI offers 27 different variables that relate to intervention outcomes and target state characteristics. The MIP dataset uses some of these measures, coding these variables for all new cases.
The Armed Conflict Database (ACD/PRIO) lists armed conflicts with at least one nation-state involved, but only includes post-1945 instances and has few cases of US involvement (Pettersson and Wallensteen 2015; Gleditsch et al. 2002).
The Military Intervention by Powerful States (MIPS) dataset, while using a narrow timeline of 1945–2003 and focusing on interventions by several great powers, offers a range of conceptualizations on the “effectiveness of military force as a policy instrument,” which MIP has adapted to code related variables for its case universe of interventions (Sullivan and Koch 2009).
RAND’s US Ground Intervention Dataset (RUGID), from 1898 to 2015 is one of the most recent efforts to document US military interventions and outcomes (Kavanagh et al. 2017). Yet RUGID only includes larger US ground interventions, which decreases and biases the sample. RUGID includes 145 cases from 1898 until 2016 in its second iteration (Kavanagh et al. 2017; Kavanagh, Frederick, Stark, Chandler, Smith, Povlock, Davis, and Geiss 2019).
Lastly, we are aware of the International Crisis Behavior (ICB) dataset, but this dataset reports on 66 to 72 crises that involve the US (depending on the version), and it focusses on a related but different unit of analysis, crisis, not military intervention (Brecher et al. 2020).
Table 1 below offers a comparison between all main datasets discussed, including definitions, temporal scope, and the number of cases that overlap with MIP, as well as the number of unconfirmed cases as per MIP case study research.5
In comparison to existing datasets, MIP doubles the universe of cases by expanding the range of definitions of intervention and exploring more sources of documentation, confirming additional interventions via extensive historical research. At least three scholarly sources confirm each case. Comparative analysis reveals that the Militarized Interstate Disputes (MID) (Palmer et al. 2015; Gibler 2018) dataset contains only about 65% of the cases found in MIP, many of which we have supplemented to include updated case study measures. Additionally, MID contains 324 dyadic cases of US military intervention, but we report only 268 cases in MID as direct US military interventions with at least a threat of force. Out of the 268 total MID cases of US intervention with some level of hostility, MIP was only able to confirm 186 of them. The CRS data, which is the most comprehensive list of US military interventions since 1776, still includes fewer confirmed cases of intervention than MIP. The remaining datasets offer few cases relative to MIP due to their post-1945 timeframes and/or limited scope conditions.6
MIP-MID Empirical Comparisons
MIP expands the universe and variables on US military interventions, while also offering the gold standard for case documentation via written case studies of each instance of US military intervention. This methodological approach allows MIP to add more cases, edit missing or incorrect data from other sources, remove cases that do not fit the definition of US military intervention, and incorporate more variables. As a result, MIP reveals distinct new patterns of US military intervention, which are vital to theoretical debates and contemporary policy debates.
In this section, we compare important patterns of US military interventions across the MID and MIP datasets. We chose MID because it is the most comparable to MIP, has the best documentation, and is most widely used by scholars. Nevertheless, it has limitations that obscure important trends in US foreign policy. As will become apparent, MIP and MID disagree on several trends, from the number of interventions to the location and nature of those interventions.
As outlined in Table 1 above, MIP contains a total of 392 cases of U.S. military intervention while removing 178 cases that were found in other datasets but could not be confirmed as U.S. military intervention by our in-depth case study analysis.7 MID contained 134 of these removed cases, but 52 of them were already correctly marked by MID as characterized by zero U.S. hostility, meaning that the U.S. did not respond to another country’s use of force. Therefore, only 82 MID cases were removed from MIP due to the inability to confirm a U.S. response. In other words, of the 268 total MID cases of US intervention with some level of hostility, MIP confirmed only 186. Even with these cases removed, MIP contains 200 more cases of intervention than MID.
MID’s case universe makes it seem as though the US only heavily relied on military might during the Cold War, but our data reveal otherwise, as shown in Figure 1.
When comparing the temporal composition of the MIP and MID case universe, MID contributes a relatively lower number of cases before 1956 and after 1989, but a larger number of cases in the 1980s. However, as documented in our codebook and case studies, we believe that many of the MID Cold War cases represent unconfirmed cases of intervention, instances of improper disaggregation of operations, and examples of US non-intervention during a dyadic intervention.
Military intervention project offers more cases within the US military intervention universe, which allows it to capture cycles of US interventions that have gradually increased in intervention frequency across time, especially in the 1980s and beyond. MID, on the other hand, reveals no such cyclical patterns over time.
Figure 2 compares our new MIP data universe to the existing MID universe of US military interventions across all eras.8 Overall, MID and MIP follow similar macro trends when it comes to region and era, but MID focuses more so on European disputes and due to the time scope, it does not reflect post-9/11 military trends.
This comparison across eras reveals that MID overreports interventions during the Cold War and underreports for all other periods, relative to MIP. Both datasets report the Cold War era (1946–1989) as the most militaristically active for the US, with the 1868–1917 era following close behind. However, MIP also reveals the post-9/11 era, running from 2001 to 2019, to be the third most active for US interventions. MID data prompt the opposite conclusion – that the US has decreased its frequency of interventions from 1990 onward.9
In addition, due to MID’s coding of only the target state of US intervention and not the region or country where the intervention took place, the dataset overreports European engagements relative to other regions of the world, as shown in Figure 2. According to MIP, the US has undertaken 34% of its interventions against countries in Latin America and the Caribbean; 23% in East Asia and Pacific; 14% in the Middle East and North Africa; and only 13% in Europe and Central Asia. In contrast, MID shows that the US has undertaken 26% of its intervention against countries within Europe; 33% in Latin America and the Caribbean; 24% in East Asia and Pacific; and 17 percent in the Middle East and North Africa. Moreover, while MIP shows that over nine percent of US interventions have occurred in Sub-Saharan Africa, MID barely registers any activity within these countries. In other words, MID does not capture one of the most important regional trends that arose in the 1990s and 2000s.10
MID displays more European conflicts perhaps due to its Cold War focus. This result could also stem from MID’s country coding system, which codes conflicts that occur within Latin America or the Caribbean as interventions that targeted European states whenever the US intervened in a related European conflict. MIP instead codes the target state, country, and region of intervention. Region is based on the physical target state/territory of the military intervention, and the MIP dataset adds a separate variable on whether the said target state was the primary target of the intervention. When including the unconfirmed cases of intervention collected by MID (n = 82), we see an even stronger focus on interventions against countries in Europe and Central Asia and countries in Latin America, while other regions remain underrepresented.
Consequently, MIP reveals that recent US interventions have increasingly targeted the Middle East and North Africa (MENA) and Sub-Saharan Africa. In a short amount of time, these interventions now make up over one quarter of total US military interventions across history. This regional expansion of the usage of force also parallels the US’s rise to global hegemon after the end of the Cold War.
Beyond frequencies of intervention, both MID and MIP datasets rate each country’s response on a scale from 1 to 5, from the lowest level of no militarized action (1), to threat to use force, display of force, use of force, and, finally, war (5). In some instances, states respond, but in others, they do not. MIP illustrates a far higher number of instances in which the US relied on the use of force than MID, more than two times more often, with MIP measuring 160 instances of direct usage of force and MID measuring 76.
Within the MIP dataset, almost half of the coded U.S. military interventions have included the direct usage of force abroad (41% of the cases) and over half of them (52%) have included displays of force. Threats of force amount to only four percent of the MIP case universe.
MID, on the other hand, shows that the US has predominantly relied on displays of force, and less so on the direct usage of force abroad. Also important to note is that within MID’s case universe, about 30 percent of the incidents marked as US military interventions do not present with any threat, display, or usage of force by the US after a detailed case study analysis – thus removing them from the sample altogether. Detailed data and additional graphics are available in the supplemental appendix.
Interestingly, the post-9/11 era, running from 2001 to 2019, appears to be the third most active for US interventions of relatively higher hostility levels. In this era, threats of force are absent, while usages of force are overwhelmingly commonplace. Since 2000 alone, the US has engaged in 30 interventions at level 4 (usage of force) or 5 (war). The post-Cold War era has produced fewer great power conflicts and instances in which to defend vital US interests, yet US military interventions continue at high rates and higher hostilities. Thus, this militaristic pattern persists during a time of relative peace, one of arguably fewer direct threats to the US homeland and security
We further visualize comparative hostilities between the US and the target state (State B) through the measurement of the highest military action taken by each side during the year of the intervention, a category that breaks down the more general hostility levels we introduced above.11 For some context, in the graph below, the measure of 1 equates to no military action, while 14 (the highest in the graph of era averages but not in the full sample of interventions) denotes a border violation usage of force. The measure 7 represents a show of troops, 9 a show of ships, 10 denotes an Alert show of force, and 12 denotes a Mobilization in the show of force broader category. Higher military actions such as clashes (18), raids (19), and interstate wars (23) exist across the spectrum of US interventions as our codebook shows, but they are not represented as averages across the eras.
As per MID’s measures, the US appears much less aggressive in its interventions prior to the Cold War. But both MIP and MID showcase the same general patterns of hostility escalation since 2001. Figure 3 reveals a widening gap of military actions between the US and State B, especially since 2001. While the US has always relied on military force, it generally paralleled its rivals’ levels of hostility until the end of the Cold War. Afterward, the US began to escalate its hostilities as its rivals deescalate it, marking the beginning of America’s more kinetic foreign policy. While MID showcases similar macro trends, its timeline and coding still prompt different patterns of intervention overall. Nevertheless, there is a widening gap between US actions relative to its opponents.
An Application of Military Intervention Project to theoretical Debates
As shown above, MIP’s expanded universe of US military interventions prompts distinct, often contrasting results from MID, the current leading dataset on military interventions. Beyond this, MIP also offers a wide range of variables related to military interventions, including measures of US national interests, human rights, intervention outcomes, and domestic variables. Below, we assess theoretical expectations in international relations, further showcasing MIP’s applicability to debates in the field. In a cursory fashion, we first consider the relationship between US interventions and the pursuit of national interests, as per the theory of realism. Then we evaluate the relationship between democratization, institutions, and US interventions, as per liberalism. Lastly, we offer data on the historical objectives of US interventions to examine the importance of humanitarian concerns.
According to realist scholars, geopolitical national interests usually underlie forceful interference in state sovereignty (Carr 1939; Morgenthau 1948; Buzan 1996). This literature equates national interest to the maintenance and global expansion of geopolitical influence, a favorable distribution of power, and the pursuit of political interests (Bellamy and William 2005; Gilligan and Stedman 2003; Perkins and Neumayer 2008; Neack 1995; Huth 1998). States may intervene to support an ally, block a regional power, or counterbalance another state’s intervention. Unless vital interests are at stake, realism predicts that states will rarely militarily intervene as they risk high economic, political, and human costs (Buzan 1996).
Scholars measure geopolitical national interests through a variety of indicators. Contiguity is a common dimension in measuring these geopolitical interests. As Souva and Prins (2006) show, the more land borders a state has, the more likely it is to employ military power. Indeed, as realists would predict, most third-party interventions originate from the same region as the target state – and in such cases, the prevention of regional diffusion of conflict or refugees appears to be the main goal (Mullenbach and Matthews 2008). Interventions do seem to impact the likelihood that conflict will spread to nearby regions. Peksen and Lounsbery (2012) show that pro-government interventions reduce the spread of conflict while pro-rebel interventions increase it.
Alliances or rivalries also interact with territoriality, limiting leaders’ ability to pursue accommodation in place of military force (Colaresi and Thompson 2005). Defensive alliances and those that settle territorial disputes can serve to deter conflict and prolong peace (Fang, Johnson, and Leeds, 2014; Gibler and Vasquez 1998). In contrast, neutrality and offensive pacts seem to bolster the risk of violence between the involved states (Leeds 2003). Moreover, alliances serve as a proxy for existing strategic interest within a country.
Differences in state power and capabilities, as measured via the Correlates of War Composite Index of National Capabilities (CINC) scores, also drive patterns of military intervention, according to realist scholars. Indeed, De Nevers (2007) finds that great powers overwhelmingly target weak states, relative to protected, strong, or defensible states. For protected states, the strength of their alliances influences whether they are targets of a great power intervention. These findings bolster realist assumptions that power dynamics drive patterns of military intervention. Yet inconsistencies linger as some studies show that disputes between states of symmetrical power capabilities tend to escalate at higher rates (Choi and James 2016; Pickering and Mitchell 2017).
Lastly, another realist proxy for geopolitical national interests is former colonial history. Powerful countries are more likely to militarily intervene within their former colonial spheres (Pickering and Mitchell 2017). Stojek and Chacha (2015) further show that colonial linkages increase the chances of intervention even when accounting for many other shared factors between the intervener and the target country, including trade and language.
It is interesting to note that one study shows a much more simplistic view of the strategic argument – one that reverses the causality of several claims above. Fordham (2004) argues that greater military capability alone has made US elites more eager to militarily intervene internationally. In other words, the US’s large investment in its military might has created path dependencies that now entice the US to intervene further.
To elucidate the causes and consequences of intervention, MIP measures several national interests-related variables from 1776 to 2019. We apply a National Interests Index across the eras that adds up separate measures on contiguity, colonial history, alliances, and natural resources.12 This additive index contains relative measures of geopolitical importance between country-dyads, including factors such as geographic continuity, shared alliances, colonial history, and the presence of oil and gas.13 We calculate the ordinal index, using State B target data, as follows:
OilDummy + ColonialDummy + AllianceDummy + ContiguityDummy.
As illustrated in Table 2, the US involved itself in military conflicts with high national interests until the 1860s, usually fighting to preserve the new nation and expand its domestic territory and sphere of influence. National interests dipped in subsequent eras during the time of the Banana Wars and Mexican Revolution but then spiked during the Cold War alongside intervention frequency.
In the post-Cold War era, the US has intervened in pursuit of fewer vital national interests as geopolitical rivalries and vital threats to homeland security have faded. The post-Cold War era has seen the US wield its military might toward more missions of democratization, human rights enforcement, humanitarian interventions, and third-party interventions in internal domestic crises across the world. The US has more frequently responded to existing crises as a third-party actor. Table 2 reveals that the US has experienced higher intervention frequencies and rates per year even when lower levels of interest were at stake.
The rates of intervention across eras are particularly interesting as the annual rate dramatically rose during the Cold War, and then only continued to grow — doubling in the post-Cold War era onward — while National Interests declined. In Tables 3 and 4 below, we highlight the result of two ANOVAs with the Bonferroni multiple comparison tests to check whether the differences in means of intervention frequencies and national interest averages across eras are statistically significant. The Scheffe and Sidak multiple comparison tests, which reveal the same patterns of statistical significance, are available in the supplemental appendix (Tables 13 and 14) alongside descriptive statistics and graphs.
Analysis of Variable
As Table 3 shows, we find statistically significant differences in both intervention frequency and national interest index across the eras.
In Table 4, we further see that when comparing the means of intervention frequency, later eras are significantly more interventionist in frequency than previous eras, with post-WWII eras showcasing statistically significant increases.
The only non-significant change arises from the mean difference between the 1990-2000 and 2001-2019 eras, but this could be due to limited intervention data in the 2001-2019 years. When comparing the means of the national interest index, we see statistical significance only between the 1990-2000 and 2001-2019 eras relative to the rest, with lower national interests in comparison to Cold War and pre-Cold War eras. These comparison tests confirm the patterns seen in the descriptive statistics of Table 2.
Table 2 presents another puzzle, however. Since the beginning of the Cold War, the US’s share of material power capabilities in the system has been gradually declining, while its interventionism in disputes of lower national interest has increased. This decline in US power capabilities as measured by the CINC index14 may be attributed to the Soviet Union’s rise in power after 1945, as this factor alone denies the US a large portion of resources in the international system. An alternative explanation is that such an indicator of traditional national material capabilities fails to capture new sources of power during the Cold War era not linked to the production of metals, energy consumption, population, urbanization, or military personnel or expenditures.
Realist accounts thus stand conflicted. Defensive realists would not predict the US’s increasing usage of force and hostilities in the post-Cold War, given fewer vital security threats and the lack of peer competitors (Taliaferro 2000; Waltz 1979; Art and Jervis 1973). Offensive realists cannot explain why the US intervenes so frequently in regions without an aspiring regional hegemon (Mearsheimer 2001). Nevertheless, realists of all stripes would note that the absence of a peer competitor is what enables the US to intervene so widely across all regions of the world.
Liberal Institutions, Interdependence, and Norms of Human Rights
Military intervention project also incorporates several factors that allow us to test liberal alternatives to realist explanations (Deudney and Ikenberry 1999). From the liberal perspective, foreign military interventions reflect international moral obligations, especially for democratic states (Lischer 2005; Walker and Pearson 2007; Herbert 2005: 30). Such liberal perspectives have become more commonplace since the end of the Cold War, advancing the stance that states — absent vital security interests — are justified and even expected to launch multilateral military interventions abroad in response to humanitarian catastrophes (Talentino 2005; Hoffmann 1996; Walzer 1977). Moreover, democratic governments are likely to export liberal values through multilateral humanitarian military interventions (Lebovic 2004; Doyle 1997; Russett 1994). With a greater focus on international institutions, liberals also view interdependence as a key factor in state behavior. Indeed, some studies show that interdependence reduces the likelihood of interstate force, especially when a large portion of states’ trading is intra-industrial (Kinne 2012; Peterson and Thies 2012). Yet in situations of trade asymmetries and export similarities, the use of force between trade partners increases (Gartzke and Westerwinter 2016; Chatagnier and Kavakli 2017).
A leading constructivist account of state behavior, Finnemore (2003) contends that neither realist nor liberal models of international relations account for observable trends of military intervention. Realism fails to explain the evolution of the full range of intervention, from unilateral debt-collecting military missions to humanitarian multilateral missions, which do not match changes in polarity or power distribution of the interstate system. Liberalism is also ill-equipped to explain how illiberal, non-democratic states tend to follow similar norms regarding intervention behavior (Finnemore 2003: 52-56). Furthermore, idealist or normative perspectives cannot account for the lack of intervention during the Rwandan genocide. Ultimately, constructivists see norms of human rights — though often co-opted and abused — encapsulated by the Responsibility to Protect as driving US military interventions in regions of internal violence (Choi and James 2016). Thus, any empirical assessment of US military interventions must also include ample human rights and institutional context (Lyon and Dolan 2007).
As Table 5 shows, in the early 1990s, the US moved from unilateral interventions to waging mainly multilateral ones, often sanctioned by the UN.
The US briefly reverted to its old unilateral ways from 1997 to 1999 as the UN could not reach a consensus on many humanitarian interventions that the US and other Western actors pushed for. In the post-9/11 era, unilateral interventions made a comeback at the expense of the new trend of multilateral military interventions that emerged in the past. More details on such trends are available in the supplemental appendix.
Below, we trace the levels of democracy across targets of US intervention in Figure 4.
Contrary to liberal expectations, since 2000, the US has intervened in countries with higher levels of democracy, as per Polity scores ranging from +10 (full democracy) to −10 (full autocracy).15
We must also ask, what objectives has the US pursued when it has decided to use force abroad? Do these objectives lend any support to the human rights argument for intervention? MIP reveals that a wide range of motivations has prevailed. It is important to note that these objectives are not mutually exclusive, and that the US can and does intervene for more than just one objective at a time. In fact, we code most interventions with multiple overlapping objectives. Only 38.78% of intervention cases present with only one main objective. For instance, in the sample case studies given in our codebook, we code Operation Restore Hope (UNITAF) in Somalia from 1992 to 1993 as Social Order and Protection and Humanitarian Intervention because UN Security Council Resolution 794 authorized the US to use all necessary means to create a secure environment for humanitarian relief operations in Somalia. In the second case study example of the Greek Civil War intervention from 1948 to 1949, we code the objectives as Maintain/Build Foreign Regime Authority because the US sought to defend the Greek state from Communist rebels. We also code the case as Protect own Military and/or Diplomatic Interests since the US also sought to ensure that Greece remained within the Western sphere of influence and could serve as a bulwark against Soviet influence and Tito’s Yugoslavia. All intervention data includes a case study such as these, with details as to why objectives (as well as other variables such as outcome and costs of intervention) were coded the way they are in the dataset.
The objective breakdown is as follows:
• Territorial objectives occurred 58 times (15% of military interventions)
• Social protection: 130 times (33%)
• Removal of a Regime: 23 times (6%)
• Protection of US’s own citizens and diplomacies: 192 times (49%)
• Policy change: 36 times (nine percent)
• Empire: 48 times (12%)
• Economic: 126 times (32%)
• Building of Regime: 24 times (six percent)
The objective to Protect the US citizens, diplomats, embassies, and property abroad during a crisis was the most frequent objective of US interventions, followed closely by Social Protection and Economic Objectives. Social Protection involves the protection of a socio-ethnic faction in the target country, general protection of civilians from human rights abuses via a humanitarian intervention, the restoring of social order in a crisis, or the suppression of fighting between armed groups.16 But in the post-WWII era, the Building of a Foreign Regime rose as of the top three objectives of US military interventions, as graphics in the supplemental appendix reveal.
Therefore, Social Protection interventions became much more common after the Cold War while Protecting our Own Diplomatic Personnel and Property, Building Regime, Removing Foreign Regime, Territory, and Policy Change objectives dominated as a proportion of objectives during the Cold War era. Nevertheless, humanitarian objectives are relatively rare.
These data snapshots speak to the incomplete explanations offered by different theoretical traditions and to the importance of era-specific trends of intervention. With its wealth of cases, both in quantitative and qualitative format, as well as a range of related variables, MIP can begin to untangle the many puzzles inherent in the phenomenon of US military interventionism across time and space, including both domestic and international dimensions.
Preliminary results from MIP show that the US has increased its military usage of force abroad since the end of the Cold War. Over this period the US has preferred the direct usage of force over threats or displays of force, increasing its hostility levels while its target states have decreased theirs. Along the way, the regions of interest have changed as well. Up until World War II, the US frequently intervened in Latin America and Europe, but beginning in the 1950s, the US moved into the Middle East and North Africa (MENA). By the 1990s, it doubled down on MENA and directed its focus to Sub-Saharan Africa and South Asia as well.
As a result, MIP highlights different trends of intervention compared to the currently leading dataset on US military intervention, the Correlate of War’s (COW) Militarized Interstate Disputes (MID) dataset. These distinct and previously overlooked trends contribute important insights to both international relations theoretical debates and policy-oriented discussions on the future of US grand strategy.
This article introduces the MIP dataset, its key trends and contributions, and potential range of future research questions. As we further develop MIP, we will strive to discern the conditions that lead to interventions as well as the consequences of those interventions on both the US and the states that the US intervenes in. We will further assess the explanatory power of main international relations theories through the indicators introduced above, as well as many others such as Cold War versus post-Cold War polarities, using robust longitudinal models. Moreover, we are interested in measurable direct costs (human and economic), as well as the unintended consequences of US interventions. Measuring those will entail statistical analysis, as well as historical analysis for how the different cases and periods of intervention informed one another. For example, what were the longer-term costs and unintended consequences of the intervention in Afghanistan and how did that intervention influence US engagements in Iraq, Libya, Syria, and Yemen?
We intend that the MIP data set and the analysis that follows provide an important resource to those interested in understanding the dynamics of US interventions historically and into the future. We contend that better data will lead to better theory testing, and ideally better policy formulation, on the subject of US military intervention.
Sidita Kushi and Monica Duffy Toft. The Military Intervention Project (MIP) would not have been possible without the support of our team at the Center for Strategic Studies (CSS), the Fletcher School of Law and Diplomacy at Tufts University. We’re especially grateful to Nils Hägerdal, who directed the final year of MIP. We are also grateful to all past and present CSS postdoctoral fellows Karim Elkady, Benjamin Denison, and Stephen Moncrief for their research support as well as their review of article drafts. Past and present CSS Research Fellows David Kampf, Lima Ahmad, Zoltan Feher, and Neha Ansari supported this research via case study research and review of article drafts. We thank past and present CSS program coordinators Anna Ronell and Polina Beliakova for their work on the Center’s research initiatives, grant management, amongst many other efforts. A wonderful team of research assistants from the Fletcher School of Law of Diplomacy were instrumental in researching MIP cases, taking notes on case studies, and backlogging data. We are grateful for our RA leaders and researchers Bryan Cassella, Mario Zampaglione, Dylan Farley, Andrew Hogan, and Emily Kennelly. We also thank the rest of our superb RA team for their hard work, including our senior RAs Daniel Creamer, Jacqueline Faselt, Moriah Graham, Daniel Morton, and Nate Dobbin, as well as: Samuel Carletta, Emilio Contreras, David Folsom, Ian Franckling, Camille Freestone, Becki Glawe, Rachel Goretsky, Michael Gradus, Sam Green, Nicholas Guarnaccia, Chelsea Hanlon, Madeleine Herr, Grady Jacobsen, Alex Kersten, Abby Kukura, Jack Lashendock, Rachael Lew, Claire McGillem, Meagan Reid, Stefan Shover, Carson Thal, Michael Vandergriff, and Cassandra Zavislak. We would like to thank the Charles Koch Foundation for its generous funding of the MIP dataset and project. Lastly, we’re thankful to the reviewers and editor of the journal for their insightful feedback.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the Charles Koch Foundation.
Sidita Kushi https://orcid.org/0000-0003-0261-4631
Monica Duffy Toft https://orcid.org/0000-0001-5550-292X
Supplemental material for this article is available online.
These numbers originate from the Military Intervention Project (MIP) Dataset.
MIP will extend its case universe to drone warfare as a separate sub-sample of US intervention type. We are currently assessing how to aggregate our individual country-year drone strikes data in a way that parallels traditional and covert military operations. Our source is the Bureau of Investigative Journalism’s Drone Warfare Database, https://www.thebureauinvestigates.com/projects/drone-war.
MIP also includes US military interventions targeting over 100 American-Indian nations during the era of the Frontier Wars, although we do not explore these wars within this paper. This is because these units of analysis differ from later US interventions and bias the sample toward frontier war dynamics given the large number of conflicts. We plan to explore these Frontier War dynamics separately. The source for these data is Friedman, Jeffrey A. 2015. “Using Power Laws to Estimate Conflict Size,” Journal of Conflict Resolution, Vol. 59, No. 7: 1216-1241.
Note that the CRS document (Salazar Torreon 2017) is not a machine-readable dataset but a list of US interventions with aggregations of multiple interventions within each year.
The unconfirmed cases column provides the final number of cases we were able confidently confirm with at least three sources. Should evidence become available through the release of new archives or other data sources, we nevertheless retained the unconfirmed cases in the codebook. The final column lists the total number of cases in the corresponding dataset, which may be larger than the number of cases directly included by MIP from this dataset. MIP may have included a dispute from other sources first, not pulled directly from the listed dataset.
The supplemental online documentation contains a comprehensive treatment of MIP’s data collection efforts and our codebook for detailed variable measurements, sources, uncertain cases, and fuller comparisons with other data sets.
Complete case narratives are available upon request.
MID includes other state actors within its intervention universe, but we only use the sub-sample of U.S. interventions. Additionally, the MID sample of interventions only include those up to the year 2010.
We categorize eras by significant outcomes that shaped US military aspirations and capabilities. The period of 1776–1864 sees the US rise above its colonial dependencies and isolationist beginnings to become a regional power; 1865 to 1917 denotes the pre-WWI era, with the US transforming into an imperialist power in Latin America; 1918 to 1945 marks the post-WWI through WWII period; 1946 to 1989 marks the Cold War. The US experiences its “unipolar moment” from 1990 to 2000; while the period from 2001 to 2019 is defined largely by the Global War on Terror.
See MIP Descriptive Statistics in the supplemental online documentation for more nuanced data on the regional makeup of US interventions across the eras.
Please refer to the MIP codebook for full coding on Highest Actions. We adapted this measure from the MID codebook.
Refer to the supplemental documentation for a frequency table of National Interest measures. This index is introduced by MIP, but its individual indicators derive from Correlates of War Project (a, b, c, d) and Ross and Mahdavi (2015).
Previous research highlights the geopolitical and military importance of land borders (Souva and Prins 2006), shared geographic region (Mullenbach and Matthews 2008), formal alliances (Colaresi and Thompson 2005; Fang, Johnson, and Leeds 2014; Gibler and Vasquez 1998), and colonial history (Stojek and Chacha 2015).
Refer to the codebook for comprehensive coding and definitions of the types of intervention objectives.
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