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date: 18 January 2018

Alliances and War

Summary and Keywords

Military alliances predate even the state system as a form of international cooperation, and they take on many forms. The motivations of states seeking to join, the commitment levels formalized in the alliance agreement, and degrees of institutionalization all take different forms in the literature, but these scholarly perspectives can be boiled down to a few approaches: the realist, the rationalist and formalist, the liberal or institutionalist, and finally, the constructivist arguments on alliance identities. Moreover, a common thread among the literature on military alliances is an understanding that alliances provide a wide range of services to their members, and contain more than one motivation for forming and maintaining the alliances. Given that the motivations for forming alliances are varied, especially during different threat environments, it is important to ask what the consequences are. In this vein, scholars consider two primary issues: if these alliances can fulfill their intended missions, and if there are unintended consequences which may arise and lead to undesirable results. A related issue to the study of what motivates alliances is in how well they perform in terms of cohesion. Cohesion is, roughly speaking, the capacity of an alliance to effectively carry out its goals. Finally, there are the coalitions—ad hoc multinational understandings that are forged to undertake a specific mission, and dissolve once that mission is complete. They are not wholly analytically distinct from wartime alliances, although the latter may have a greater degree of institutionalization and may predate a specific wartime operation.

Keywords: military alliances, war, international cooperation, alliance cohesion, coalition warfare, alliance motivations, alliance literature, wartime operations


Military alliances represent one of the most important forms of cooperation in the international system. Because of anarchy and self-help, states essentially have two paths to augmenting their security: arming themselves, or forming alliances (Waltz 1979). Hence, forging an alliance with another state or set of states is premier among strategies in a state's arsenal to advance its policy agenda. Accordingly, military alliances are central to the field of international relations, and their study has attracted an enormous amount of scholarly attention.

The role of alliances in shaping the system and war predates the state system. For example, prior to the Peloponnesian Wars, Athens and Sparta were allies, united in repelling a series of Persian invasions against the Greek states. Once the Persians were defeated in 479 bce, the coalition of over 30 Greek city states unraveled. According to Thucydides’ (431 bce) account, the alliance network that was created in the decades following the defeat of the Persians was instrumental in bringing about the Peloponnesian Wars. The alliance politics alone make Thucydides’ description of the wars worth reading. The relationship between alliances and war is foundational to the field of international relations, and has enduring consequences, from ancient Greece to the present day.

Toward a Definition of Alliance

Military alliances take many forms. There is variation in the motivations of states seeking to join, the commitment levels formalized in the alliance agreement, and degrees of institutionalization. These components of alliance take different forms in the literature. For example, it is possible to view the concepts of alliance, coalition, entente, and pact differently. Alternatively, it is possible to view these concepts as on one spectrum of alliance commitment level or degree of institutionalization. These issues are handled differently by authors working in the area of alliances.

Weitsman (2004:34) defines alliance as a formal or informal agreement between two or more states intended to further (militarily) the national security of the participating states. Since many of the most important historical and contemporary alliances had no formal treaty of alliance (e.g. the Triple Entente prior to World War I, Israel and the United States today), excluding from inquiry those without a formal treaty limits the scope of study. To further break down the analysis, Weitsman (2004:35) identifies six security promise levels: (1) a promise to maintain benevolent neutrality in the event of war; (2) a promise to consult in the event of military hostilities with an implication of aid; (3) promises of military assistance and other aid in the event of war, but unilateral and without preprepared or explicit conditions specified; (4) a promise to come to the active assistance of an ally under specific circumstances; (5) an unconditional promise of mutual assistance, short of joint planning, with division of forces; and (6) an unconditional promise of mutual assistance in the event of attack with preplanned command and control and the integration of forces and strategy. This is a slightly different scheme from Singer and Small (1966), who distinguish between defense pacts, neutrality pacts, and ententes. To some, the formal commitment of an alliance is key (Morrow 2000). This is captured in the Alliance Treaty Obligations and Provisions data set which codes only formal treaties in terms of the treaty content: as promises to aid an ally either offensively or defensively in the event of military conflict; promises to maintain neutrality in the event of military conflict; promises not to engage in military conflict with each other; or promises to consult and cooperate during international crises possessing potential for military conflict (Leeds and Anac 2005).

What these conceptualizations share is an understanding that military alliances provide a wide range of services to their members, and contain more than one motivation for forming and maintaining the alliances.

The Alliance Literature: Yesterday and Today

The alliance literature is rich and nuanced. While much of the work is varied, it can largely be classified into several main streams. The most dominant strand is the realist literature. Closely connected to the realist arguments on alliances is the rationalist and formal literature. A third literature, written largely in response to realist and rationalist arguments about alliances, is a less well developed one on alliances from the liberal or institutionalist perspective. Connected to these are constructivist arguments on alliance identities. Constructivist and identity-based arguments are becoming more prevalent and will likely continue to be an important research focus in the coming years. In this essay, I take institutionalist, constructivist, and identity based arguments together, although as these literatures continue to progress, important differences will become increasingly apparent.


Classical realists such as Morgenthau (1948; 1970) and Gulick (1955) argued that military alliances served to augment power for nation states and effectively prevent enemies from imposing their will on a nation state. As Liska observed, the utility of alliances derives from their ability to deter aggression from a common enemy. In his words: “alliances are against, and only derivatively for, someone or something” (1962:12). This view of alliances is pervasive in the field – both in theory and in practice.

The capability aggregation model of alliances advances the idea that military alliances increase power resources and capabilities available to a state. When a state forms an alliance, it automatically is able to add the power of their allies to its own (Morgenthau 1970:80). Similarly, neorealists, or structural realists, argued that alliances are formed because they function as security maximizing tools. States ally to enhance their security. As Waltz (1979) writes, states are concerned with maximizing security, not power. “If states wished to maximize power, they would join the stronger side, and we would see not balances forming but a world hegemony forged. This does not happen because balancing, not bandwagoning, is the behavior induced by the system” (1979:126). Structural realism speaks to when and why military alliances will form; it also explains the alliance dynamics that emerge under different system types and how central an alliance is to key member states.

Multipolar systems give rise to fluid relationships – military alliances are formed for expediency, but dissolve easily (Waltz 1979:168). In contrast, in bipolar systems where power capabilities are concentrated in just two states, few states in the system have the ability to make a difference in the balance of power. In these systems, relationships will remain more or less fixed in blocs, and the most powerful countries in the system will focus on trying to alter the balance of power through arming themselves (Waltz 1979:168). Related to the arguments about alliance behavior and polarity is the literature on power distribution and war. This, too, is a voluminous literature (see, for example, Deutsch and Singer 1964; Singer et al. 1972; Bueno de Mesquita 1978; Waltz 1979; Stoll 1984a; 1984b; Sabrosky 1985; Vasquez 1993; Mansfield 1995; Geller 2000; Weitsman and Shambaugh 2002). See also the power transition literature (e.g. Organski 1958; Organski and Kugler 1980; Kugler and Lemke 1996; 2000; Rapkin and Thompson 2004).

Subsequent elaborations of alliance theory focused on important related arguments. Walt (1987) posited that states respond to imbalances in threat, not just capabilities, adding a perceptual dimension to the capability aggregation argument. Other scholars such as Priess (1996) tested balance of threat arguments in other contexts.

Waltz's arguments and the elaborations following in his tradition have two key analytical components in common. First, above all, states seek to balance against others – alliances are formed to counter threats or imbalances in power. Second, whether states are allying in response to threats or imbalances in power, they do so to add the power of their allies to their own.

Other theoretical layers have been added to structural realism to improve its analytical power. For example, Christensen and Snyder (1990) and Christensen (1997) added insights yielded by the security dilemma to generate more insight regarding alliance dynamics. Schweller (1998) innovates balance of power theory by describing a balance of interests that drives state behavior at the unit and systemic levels. David (1991a; 1991b) argues that leaders of third world states make alliance choices based on how they can best assure their political survival. In other words, they balance, but not in response to external threats. Cha (1999) elaborates on Snyder's seminal work on the alliance security dilemma (Snyder 1984), and constructs an argument about quasi alliances. Threats and alliance behavior are mediated by the dilemma of fears of abandonment versus entrapment. These works have given defensive or neoclassical realism an important place in alliance politics, emphasizing the domestic politics of a state's alliance choices.

More recently, Weitsman (2004) argued that alliance choices were governed by the actual level of threat facing states, and that threats emanate from within alliances, and are not just external to them. She examines the effects of threats on alliance cohesion and explores the similarities and differences between the operation of alliances during peacetime and wartime. Schweller (2006), in contrast, seeks to explain why states failed to understand threats confronting them. In other words, he addresses what he calls underbalancing. Davidson (2008) explores neoclassical realist explanations of transatlantic burden sharing in contemporary American wars. Tierney (2008) further delineates chain-ganging behavior into offensive and defensive behaviors. In other words, realist approaches to the study of alliances remain a vibrant avenue of scholarship.

Rationalist and Formal Approaches to the Study of Alliances

Rationalist and formal approaches to the study of alliances constitute one of the most important avenues of this study. The scholarship in this area addresses many of the most significant questions about alliances, and has been one of the most dynamic areas of growth in our understanding of alliances. This branch of research in its inception was connected with the Correlates of War (COW) Project. The COW Project was founded in 1963 by political scientist J. David Singer, joined by historian Melvin Small. While its central purpose was to advance our understanding of war, one of the major components of the project has helped scholars understand the relationship between alliances and war. The data collected on military alliances enabled researchers to better explore the formation of alliances, their duration and their effects (for a review, see Sprecher and Krause 2006). This pioneering work has shaped not only the study of alliances and war, but international security studies more broadly.

Since the inception of the COW Project, our understanding of alliances has advanced dramatically. Some of the central questions addressed by formal theorists examining alliances focus on deterrence and signaling (Morrow 1994; Sorokin 1994; Smith 1995; Fearon 1997; Leeds 2003), the tradeoffs between security and autonomy and arming or allying (Altfeld 1984; Morrow 1991; 1993; Conybeare 1994a), and the formation, frequency, and endurance of alliances (Gibler and Rider 2004; for a review see Sprecher 2004). Rationalist and formal arguments also address issues of the reliability of allies (Sabrosky 1980; Bueno de Mesquita 1981; Conybeare 1992; Morrow 1994; Smith 1995; 1998; Fearon 1997; Leeds et al. 2000; Gartzke and Gleditsch 2004) and the relationship between alliances and war (see below, and Leeds 2003).

Some of the rationalist work straddles the divide with realism. Schweller's arguments (1994) about bandwagoning contain components of realism and rationalism, by emphasizing the costs and benefits associated with states’ alliance choices. Snyder's (1997) work on alliance formation and alliance management similarly adopts elements of realism as well as rationalism in the analysis.

Institutionalism, Constructivism, and Identity

In the aftermath of the Cold War, the durability of the North Atlantic Treaty Organization (NATO) sparked interest in examining military alliances as institutions and applying institutional theory to their study (Duffield 1992; Hellmann and Wolf 1993; Duffield 1994–5; 1995; McCalla 1996; Haftendorn et al. 1999; Wallander 2000; Long et al. 2007). These studies were essential to advancing the ideas about military alliances beyond traditional capability aggregation models.

The insights generated from the study of alliances from an institutionalist perspective are closely twined with understanding the Atlantic Alliance as something more than the sum of its parts – a security community in which the members transcend a mere alliance. The community represents shared values, shared commitments to liberal democracy, and shared world views. This perspective was first expressed as early as 1957 with Karl Deutsch’s co-authored book on political community in the North Atlantic Area. This central idea lay dormant throughout most of the Cold War, as students of alliance politics focused principally on the great power and threat divide between NATO and the Warsaw Treaty Organization. Subsequent work in the post–Cold War era further developed these ideas in important ways (Adler 1992; Risse-Kappen 1995; Adler and Barnett 1996; Katzenstein 1996; Acharya 2001; Hemmer and Katzenstein 2002). The security communities literature augments the alliance literature in two especially noteworthy ways. First, it connects scholarship on democracy, alliances, conflict, cooperation, war, and peace (see e.g. Kacowicz 1995; 1998; Bially Mattern 2005; Bjola and Kornprobst 2007). And second, it helps us understand the connection between alliances and identity (e.g. Williams 1997; 1998; Williams and Neuman 2000; Pouliot 2006; cf. Cox 2005).

Alliance Motivations

One of the most important questions the alliance literature addresses is why states ally. The answer to why ally is central to the relationship between alliances and war and indeed the function of alliances more generally. It is essential to view these motivations dyadically, since motivations must be compatible, and multilateral alliances generally represent more than one motivation (Weitsman 2004:29).


Military alliances are most commonly considered strategies in a state's arsenal to counter an imbalance of power or to combat a shared external threat (Waltz 1979; Walt 1987; Schweller 1997; Snyder 1997; on the distinction between hard and soft balancing, see Paul 2005; Brooks and Wohlforth 2005). Inherent in this perspective is the idea that alliances allow states to add the power of their allies to their own in order to deter enemies or prevail in the event of war. In contrast to pure military balancing, or hard balancing, weaker states may undertake more limited measures short of military alliances to counteract a more powerful state. This has been called “soft balancing” by some scholars (see Brooks and Wohlforth 2005; Paul 2005).

The way in which balancing behavior manifests differs depending on the distribution of capabilities in the system and perceptions of offense or defense dominance (Christensen and Snyder 1990; 1997). In multipolar systems, when states confront common threats in the system, and perceive defensive strategies as dominant, their alliance choices will give rise to buckpassing – relying on others to challenge the threatening country. According to Mearsheimer (2001), buckpassing is preferable to balancing since it is cheaper. In bipolar systems, however, states must balance against their rivals; no other state is sufficiently powerful to effectively pass the buck. According to Christensen and Snyder (1990), when states face common threats under multipolarity and there is a perception that offensive strategies dominate, chain-ganging behavior will occur; states will “unconditionally tie themselves to reckless allies whose survival is seen to be indispensable to the maintenance of the balance” (Christensen and Snyder 1990:138). Tierney (2008), elaborating on chain-ganging behavior of alliances, argues that in tight alliance systems, states may engage in offensive chain-ganging, where states are rendered more bellicose by their alliance partners, or defensive chain-ganging, in which bellicose states are restrained by their allies. The management function of alliances is an important alliance motivation which has garnered increased interest, particularly in the post–Cold War era.

Tethering: The Management Function of Alliances

While balancing is indeed a central role alliances play, it is by no means the only one. Scholars such as Schroeder (1976) sought to understand the conflict management role of these institutions. Elaborating on this perspective, Weitsman (1997; 2004) argued that a military alliance may be forged to contain an adversarial relationship; that is, states tether their enemies to them to neutralize the threat. From this point of view, alliances function as other international institutions do, to enhance transparency, increase the costs of defection, and make cooperation cheaper, and, therefore, more likely. Long et al. (2007) further examine the institutionalization function of alliances and find that it successfully keeps the peace among member states.

Viewing alliances as institutions generates insights into the ways in which threats within an alliance are managed. Thus NATO, in the words of its first Secretary-General, Lord Ismay, was originally formed “to keep the Soviets out, the United States in, and the Germans down.” Examining NATO purely as a balancing response to the Soviet Union misses the important dynamics that exist within the alliance. Both Greece and Turkey are members of the NATO alliance despite the fact that they are enemies; they are not adding the other's power to their own within the context of the alliance. Nonetheless, they may gain important institutional advantages from being allied adversaries, in transparency and conflict management function. One unintended consequence, however, of effective tethering alliances is that although they are formed to manage conflicts among signatories, they may appear threatening to nonmembers, heightening the level of threat and uncertainty in the international system. Weitsman (2004) calls this dynamic the alliance paradox, and argues that it culminates in a higher probability of war in the system.

Pressman (2008) further develops the idea of alliances as mechanisms of restraint. Some states choose alliance partners in order to prevent war from occurring. Pressman explores in detail two of the most important US alliances, the US–Israel partnership and the Anglo-American relationship, in a series of case studies.


Another alliance motivation that scholars have explored is when states seek to form an alliance to protect themselves from a very powerful and threatening enemy. Here, states bandwagon with the threatening state, capitulating to it to ensure survival (Waltz 1979; Walt 1987). Schweller (1994; 1998) persuasively argues that bandwagoning behavior may be viewed as what small states do for strategic gain, given their inability to counter the power capabilities of a strong state. While bandwagoning behavior occurs more rarely than balancing, the logic is used often to explain or justify certain foreign policy objectives. For example, the fear of “dominoes falling” uses bandwagoning logic – the belief that a hostile state will reap allies through victory culminating in a snowball effect. Realists generally posit that balancing is more prevalent than bandwagoning behavior is, although Schweller (1997) argues that it is not as uncommon as earlier realists such as Waltz (1979) and Walt (1987) maintained.


This is described by Weitsman (2004) as forming low commitment level alliances with states to draw them into one's sphere of influence. This will allow the initiating state to consolidate its power and shut off avenues of expansion to potential adversaries, while not proving overly provocative to rivals. Hedging alliances allow states more freedom of action in the international system, and may set the stage for enhanced cooperation among signatories in the future. The Partnership for Peace initiatives and the Shanghai Cooperation Organization, which includes China and Russia among its signatories, are examples. Art (2004) uses the concept of hedging to understand European security policy in the post–Cold War period. Although less well developed than other alliance motivations, under conditions of reduced systemic threat, hedging options remain important. Here, other motivations, such as ideology (e.g. Haas 2003; Owen 2005), regime type (e.g. Siverson and Emmons 1991; Siverson and Starr 1994; Werner and Lemke 1997; Peceny et al. 2002), identity (e.g. Katzenstein 1996; Williams 1997; 1998), and domestic politics (e.g. Barnett and Levy 1991; David 1991a; 1991b; Narizny 2003) may come into play as well.

Alliances as Cause of War or Cause of Peace?

Given that the motivations for forming alliances are varied, especially during different threat environments, it is important to ask what the consequences are of those alliances. Do they actually fulfill the mission they were intended to fulfill? Or are there unintended consequences that culminate in undesirable results? In many ways these are the two central questions at the heart of the literature on alliances and war. Do alliances effectively deter war and keep the peace in the system? Or do they increase hostility and insecurity in the system and foster war?

Alliances Deter War and Promote Peace

A number of important studies examining the relationship between alliances and war found that military alliances are actually more often associated with peace in the system (e.g. Singer and Small 1968; Levy 1981). The traditional realist literature that focuses on states entering alliances out of a desire to balance threats or power implies that doing so will deter aggression and prevent war from occurring in the system (Morgenthau 1948; Waltz 1979). Scholars disagree, however, on how exactly alliances foster peace. While classical and structural realists argue that they do so via balancing and deterrence, other studies have found that alliances operate as important tools of restraint for allies (Schroeder 1976; Weitsman 1997; Gelpi 1999; Vasquez and Elman 2003; Weitsman 2004; Long et al. 2007; Pressman 2008; Tierney 2008). In these studies, restraint is a byproduct of the alliance, not necessarily or always the reason for forming the alliance in the first place. There are two different mechanisms at work in these studies – the ability of alliances to keep the peace among themselves, and the ability of allies to restrain each other. In other words, when examining the relationship between alliances and peace, one needs to look not only at the extent to which alliances serve to restrain allies from courting war with others, but also the extent to which the alliance agreement itself serves to temper hostility within an alliance. For example, when studying the history of NATO and peace, one might ask the question whether or not NATO deterred war with the Soviet Union and/or the Warsaw Pact, but also the extent to which NATO deterred war between Greece and Turkey, France and Germany, or other dyads within NATO with high levels of animosity at some point in the alliance's history.

Alliances Promote War

Military alliances have also been charged with creating a more dangerous and war-prone international environment. One of the principal avenues of argument here posits that the formation of an alliance, by triggering the security dilemma, will culminate in a counterbalancing alliance, and increased alliance polarization, which in turn will heighten the possibility of misperception, hostility, and the likelihood of war (Kaplan 1957; Wright 1965; Holsti et al. 1973). Alliance commitments become extremely important and the dictates of maintaining alliance cohesion make it more difficult to resolve inter-alliance conflict. When alliances become rigid, states may drag their allies into wars they might not otherwise fight. This makes alliances even more dangerous – not only do they culminate in war, they also expand the number of states involved in it (see Levy 1981).

Arguments that address the relationship between alliances and war also examine the polarization of the system and polarization of alliances. The polarization of the system generally refers to the number of independent powers in the international system and the degree to which other states are connected to them. In other words, polarity is the number of great powers in the system; polarization is the degree to which other states adhere to those poles (Raymond and Kegley 1990:10). Arguments link different distributions of power in the system and the degree of tightness or looseness of the states connected to each pole with the onset of war or peace (Wallace 1973:577; Bueno de Mesquita 1975). Waltz (1979), for example, argues that a tight bipolar system leads to peace, since there is clarity about one's friends and enemies. This certainty lends itself to stability. However, other scholars argue that a high degree of polarization in the system actually leads to more instability (Singer 1963; Deutsch and Singer 1964). In a powerful critique of these arguments, Bueno de Mesquita (1978) argued that it was not so much the polarization of the system that mattered, but assumptions about how states act and respond under conditions of uncertainty.

The findings on alliance polarization and war are similarly mixed. Wallace (1973) finds that when polarization is extremely low and extremely high it is correlated with the onset of war, while moderate levels of polarization seem to lower the likelihood of military conflict. Bueno de Mesquita (1978) finds that in the twentieth century, increased tightness is associated with the occurrence and duration of wars. Wayman (1984), distinguishing between power polarity (states form the poles) and cluster polarity (sets of states form the poles), finds that power bipolarity and cluster multipolarity are both less likely to lead to war than cluster bipolarity or power multipolarity. Replicating Wallace's (1973) study, Moul (1993) finds no relationship between polarization and war. In large part, the differences in the findings of these scholars are reflected in the different ways they conceptualize and operationalize alliance configurations and war.

Alliances may create more security for those that form them, but heighten the insecurity of others. As a consequence of increased threat perception and hostility in the system, war may result. This is the alliance paradox – the more effective an alliance is at keeping the peace among its signatories, the more likely it is to heighten systemic insecurity and war (Weitsman 2004). Alliances promote wars in other ways as well. Under conditions of multipolarity and perceived offense dominance, states may chain-gang, and war may be the result (Christensen and Snyder 1990).

What appears clear from all of these studies is that the relationship between alliances and war (or peace) holds only under specific situations. What those conditions are becomes important to understand. For example, Leeds (2003) argues that one cannot simply posit a relationship between alliances and the onset of war; rather, the type of alliances states form provides different information to other states in the system. She finds that potential challengers will be less inclined to initiate disputes if they know their adversaries have allies committed to intervening in the event of an attack. Further, challengers with allies committed to cooperating are more likely to initiate a dispute than those potential challengers without allies (Leeds 2003:435–6). In other words, it is not just having allies or not that matters, it is the type of alliance commitment one has that affects war decisions.

The Verdict on Alliances as Causes of War

Some of the most interesting and elaborate work on military alliances takes on the question of alliances as a cause of war. Yet, the literature remains fragmented and incoherent in large part because of differences in the way the studies are constructed and implemented. The lack of consistency in the way variables are identified and measured makes it difficult to generalize from the literature to the practice of international politics. While many of these studies are extremely interesting and important, much work remains to be done to streamline and advance the alliance literature on the question of the causes of war and peace.

Alliance Cohesion

Most studies of alliance politics focus on why these partnerships form in the first place. An equally important question is: once formed, how well do alliances perform? There is, however, divergence among scholars regarding the meaning of cohesion. Chernoff (1990) implies that alliance cohesion refers to the distance between or among allies’ goals. Richardson's (1996) analysis of the relationship between the US and Britain during the Suez crisis and the Falklands War implies that cohesion has to do with differences between or among allied states’ objectives. Walt (1997) indicates that cohesion is connected to alliance duration; O'Leary (1988) writes that cohesion is about agreement and shared goals, although in the same edited volume, Ravenal (1988) indicates that cohesion is about commitment. In the most recent in-depth exploration into the causes and consequences of alliance cohesion, Weitsman (2004) adopts a behavioral conceptualization of cohesion, which explores how effectively allies are able to coordinate their goals and strategies toward attaining those goals (Holsti et al. 1973).

The ability of signatories to coordinate strategies and tactics becomes a critical determinant of its effectiveness. Effectiveness is essential for a military alliance to manifest power. For example, the cohesion of the Central Powers – Germany and Austria-Hungary – during World War I was quite limited. As a consequence, the military effectiveness of the alliance was inadequate. In contrast, the Triple Entente was relatively cohesive; as such, it was able to prevail militarily over its adversarial counterpart. Maintaining the cohesion of an alliance is essential to advancing the objectives of the signatories. It is also necessary for the successful exercise of power (Weitsman 2003).

What brings states together will affect the cohesion of their alliance. Alliance cohesion is frequently a derivative of threat levels. Threats may emanate from outside the alliance to the signatories in a symmetrical way. In this case, states will have an important unifying cause that will facilitate their coordination of goals and strategies. Thus, balancing alliances will have the highest cohesion levels of any type of alliance. If threats emanate from outside the alliance, but not in a symmetrical or uniform way, it will be harder for states to agree on goals and strategies. Some threats may be complementary, others may be divisive. When states are driven to ally with one another because of the threats they face from each other, then the principal threat to the alliance will actually exist within the alliance. In this case, cohesion will be very nearly impossible to attain and maintain. Tethering alliances, therefore, will be less cohesive than any other type (Weitsman 2004).

The determinants of alliance cohesion are different during periods of peace as opposed to war. During wartime, alliance cohesion may be reflected in how well countries coordinate and implement their warfighting strategies, how effectively command and communication structures work, interoperability, and the ability to limit friendly fire incidents.

The dictates of maintaining wartime alliance cohesion can be extremely costly. As Weitsman (2003; 2009) argues, the dictates of alliance cohesion may culminate in longer wars and extremely costly side-payments to allies in order to keep the alliance together. In 2001, the US General Accounting Office found that the requirements of alliance cohesion during the Kosovo air operation of 1999 meant departure from US military doctrine, which in turn impeded the operation and increased the risk to American and allied forces (United States General Accounting Office 2001). In other words, the effects of alliance cohesion are highly consequential; they affect the war process and have important policy implications.

Coalition Warfare

The relationship between alliances and war matters also in shaping the way wars unfold. Contemporary inter-state warfare is frequently prosecuted via coalition. Coalitions are ad hoc multinational understandings that are forged to undertake a specific mission, and dissolve once that mission is complete. They are not wholly analytically distinct from wartime alliances, although the latter may have a greater degree of institutionalization and may predate a specific wartime operation. Coalitions and wartime alliances are both subsets of multinational operations, which may include other forms of multilateral cooperation, such as peacekeeping missions (Weitsman and Balkin 2006:4–5).

There are a number of particularly important trends regarding coalition warfare in the past century. The absolute size of coalitions has grown over time; the United States in particular has increasingly used coalition warfare as its strategy to prosecute wars; and coalition warfare has been an enduring feature of the international system. In the past two centuries, the number of states involved in any one war has grown dramatically. Coalition size has exploded as the number of the states in the international system has grown (Weitsman and Balkin 2006).

Given the importance of the topic, it is surprising how few comprehensive studies of coalition warfare exist. The broader debate within the field has centered on internationalism versus isolationism, or unilateral versus multilateral strategies (Corbetta and Dixon 2004). The avenues of inquiry include whether multilateralism or unilateralism is the better approach to foreign policy (e.g. Keohane and Nye 1985; Urquhart 1986–7; Gallarotti 1991; Lake 1992; Ruggie 1993; Stewart and Forman 2002). Other scholars have addressed the question of whether or not a multilateral framework enhances the legitimacy of action (e.g. Barnett 1997; Hurd 1999; Cronin 2001; Johnston 2001). The most important work on coalition warfare has explored the way coalition politics have played out in specific wars (e.g. Bennett et al. 1994; 1997; Daalder and O'Hanlon 2000; Lambeth 2001; Martin and Brawley 2001; Bensahel 2003; Weitsman 2003; Biddle 2005/2006).

Coalition operations add a degree of complexity to the war process. Burden sharing issues become paramount (Olson and Zeckhauser 1966; Sandler and Hartley 1999; Weitsman 2008). The prerequisites for developing a coherent multinational fighting force require enormous planning. The harmonization of military strategy is complex in design, even more so in execution. States must use common or compatible doctrine, tactics, techniques, and procedures, which require a significant amount of coordination. In other words, there must be interoperability – not just in terms of weaponry, but also in terms of language, communications, doctrine, and the exchange of information. Planning for interoperability requires a considerable degree of familiarity with one another's commanders and staff, visits, the creation of liaison teams, multinational training exercises, and an assessment of the logistical interoperability among partners (Weitsman 2009). The multifaceted nature of multinational operations reduces the speed and flexibility in responding to any action on the ground, and reduces autonomy in action. This can severely hinder wartime operations, as it did in NATO's war in Kosovo. The cumbersome decision making structure in the conduct of that war made it a “war by committee.” The alliance was less efficient and effective as a consequence as it prosecuted the war in the former Yugoslavia (Bensahel 2003).

There is no doubt that coalitions and alliances affect the war process (Weitsman 2003; 2004). The needs of alliance and coalition cohesion may alter the way in which a war is fought (United States General Accounting Office 2001). Coalition dynamics may affect the length of a war and the ease or difficulty of terminating it, and its cost in human and material terms (Weitsman 2003; 2008; 2009). Although the conventional wisdom about coalitions and alliances is that they are formed in order to aggregate power capabilities, increase the likelihood of victory, and decrease the costs of war, in reality the opposite is often true. Coalition partners and allies may actually increase the costs of war because of the requirements of interoperability, and decrease the likelihood of winning because of the dictates of alliance cohesion. In other words, allies and coalition partners may actually serve as strategic liabilities in wartime (Weitsman 2003; 2006). The dynamics of alliances and coalitions on the war process are complicated and consequential. This remains a critical avenue of research and exploration.

In Lieu of Conclusions: Future Directions

Few areas of research in the area of international security matter as much as the study of military alliances. Both the causes and consequences of states’ alliance choices have a profound effect on the operation of the international system. While an enormous amount of excellent work has been done, there are a number of areas that could use attention.

First, there is less than optimal consistency between the qualitative and quantitative work. New scholarship on military alliances should attempt to unify these research enterprises in order to advance the literature in productive ways. Bridging the gap between these two important research areas will allow for the delineation of the most essential patterns in the system. Striving for consistency in operational measures will facilitate scholars’ abilities to speak to one another in terms of their findings. Even scholars who address the same question – such as, do alliances promote war or peace? – have different answers, as the operationalization of the variables differs from study to study.

Second, although the question of what gives rise to alliances in the first place has been explored extensively, less well studied is how alliances function once formed. While newer studies explore alliance efficacy and cohesion, the policy implications for contemporary alliances still require investigation. Further, studies need to distinguish between the peacetime and wartime operation of alliances. The norm of coalition warfare among advanced industrialized nations has become increasingly institutionalized. Has this changed the face of warfare today? Do coalition operations differ from wartime alliance functions? What are the prerequisites of interoperability? Do these requirements affect or impinge on the sovereignty of nation states? How does fighting via coalition alter the war process itself? Is war termination easier or more difficult? Does having an international constituency that requires attention alter the way in which wars are prosecuted? These are all questions that take on extremely important and complex issues of alliance politics and war. Seeking answers to these questions will advance the international relations literature, and help guide policy makers in the future.


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I am grateful to Theo Farrell, David L. Hoffmann, and two anonymous reviewers for their excellent feedback on this essay.