Emerging Trends in The Social and Behavioral Sciences · Why Do States Sign Alliances?

Why Do States Sign Alliances?

Item

Title: Why Do States Sign Alliances?
Author: Leeds, Brett Ashley
Research Area: Social Processes
Topic: International Relations
Abstract: Despite the fact that policy‐makers and scholars of international politics have often expressed skepticism about the efficacy of international agreements, formal military alliances have been an important feature of international politics for centuries. This essay first introduces the dominant explanation for why states sign alliance treaties: state leaders use formal alliances to convey credible information about their future intentions to partners and adversaries. It then considers empirical evidence in support of this perspective, particularly with regard to deterrence and compliance. Following this summary, I raise five puzzles that contemporary researchers are working to resolve, but which leave room for further analysis and development. First, I discuss the challenges faced by large n empirical studies of alliance formation. Next, I ask why strong states ally with weak states and consider some of the most compelling recent explanations. Third, I consider the complicated relationships between alliances and war. Fourth, I examine how alliances affect cooperation among member states. Finally, I encourage scholars to continue a recent focus on how alliances are designed. While we have seen significant progress in understanding military alliances over the last 20 years, primarily because of the development of game‐theoretic models that capture strategic interaction and the collection of new data that allow for nuanced tests of the hypothesized relationships, there is good reason to believe that we will continue to see significant innovation over the next decade.
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Identifier: etrds0387
extracted text: Why Do States Sign Alliances?
BRETT ASHLEY LEEDS

Abstract
Despite the fact that policy-makers and scholars of international politics have often
expressed skepticism about the efficacy of international agreements, formal military
alliances have been an important feature of international politics for centuries.
This essay first introduces the dominant explanation for why states sign alliance
treaties: state leaders use formal alliances to convey credible information about
their future intentions to partners and adversaries. It then considers empirical
evidence in support of this perspective, particularly with regard to deterrence
and compliance. Following this summary, I raise five puzzles that contemporary
researchers are working to resolve, but which leave room for further analysis and
development. First, I discuss the challenges faced by large n empirical studies of
alliance formation. Next, I ask why strong states ally with weak states and consider
some of the most compelling recent explanations. Third, I consider the complicated
relationships between alliances and war. Fourth, I examine how alliances affect
cooperation among member states. Finally, I encourage scholars to continue a recent
focus on how alliances are designed. While we have seen significant progress in
understanding military alliances over the last 20 years, primarily because of the
development of game-theoretic models that capture strategic interaction and the
collection of new data that allow for nuanced tests of the hypothesized relationships,
there is good reason to believe that we will continue to see significant innovation
over the next decade.

INTRODUCTION
Alliances are formal agreements among independent states to cooperate in
the event of military conflict. Alliances are created in writing, and the written
documents specify the obligations of the member states and the conditions
under which those obligations are invoked. Thus, alliances represent formal intergovernmental cooperation. The promises incorporated in alliances
include at least one of the following: (i) a commitment to assist an alliance
partner militarily in the event of conflict with an outside state; (ii) a commitment to remain neutral and refrain from assisting a member state’s adversary
in any way, should conflict occur between a member state and an outside
state; (iii) a commitment to consult with the goal of producing a coordinated
Emerging Trends in the Social and Behavioral Sciences. Edited by Robert Scott and Stephen Kosslyn.
© 2015 John Wiley & Sons, Inc. ISBN 978-1-118-90077-2.

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response to a military crisis that might arise in the future (Gibler & Sarkees,
2004; Leeds, Ritter, Mitchell, & Long, 2002).
Traditionally, scholars have been quite skeptical that formal intergovernmental agreements matter in international relations, particularly in the
“high politics” arena of national security, often citing German Chancellor
Bethmann-Hollweg’s pre-invasion pronouncement that the Guarantee of
Belgian Neutrality was a “mere scrap of paper” (Cooke & Stickney, 1931, p.
382). Thus, scholars have wondered why it is that policy-makers choose to
codify their alliance promises in formal agreements and whether military
alliances have any effect on state behavior or international outcomes. In this
essay, I review our current understanding of why state leaders choose to
formalize their alliance commitments, the effects formal alliances have on
various aspects of international behavior and outcomes, and what I see as
some important remaining puzzles that scholars continue to study.
FOUNDATIONAL RESEARCH: WHY DO STATES FORMALIZE
ALLIANCE COMMITMENTS?
While scholars have long discussed “alliances” as crucial to understanding
international politics, many traditional approaches used the term differently
than we use it here. Balance of power and balance of threat theories, for
instance, focus on alignments rather than formal alliances (e.g., Walt,1987,
Waltz, 1979). Snyder (1997, p. 6) defines alignments as “expectations of states
about whether they will be supported or opposed by other states in future
interactions.” States that are believed to share common interests and pursue
similar goals in the international system are thus aligned. For the purpose of
this essay, however, they are only allied if they have signed a formal agreement committing them to cooperate in future conflicts with outside states.
As Morrow (2000) helpfully points out, aligned states often fight together
in wars, and it is certainly not necessary to have a formal agreement as the
basis for military cooperation—consider, for instance, the recent emphasis
among policy-makers on creating “coalitions of the willing,” which arise in
response to particular policy issues. So, what benefits do states gain from
formal alliance agreements?
A key reason that states formalize their alliance commitments is to provide
information to potential adversaries about their intention to cooperate. Formal agreements convey this information more credibly than simple verbal
statements because of the costs they impose on their members. Alliance members incur costs of negotiating and formalizing their agreement and of establishing means of coordinating their policies effectively (e.g., through official
meetings among military officers, consultation on foreign policy issues, and
joint training exercises). Alliance members also experience costs if they break

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a formal agreement. If the alliance is invoked and a member state chooses
not to fulfill its commitment, its international reputation for reliability will
suffer and the leader may suffer domestic audience costs as well (Crescenzi,
Kathman, Kleinberg, & Wood, 2012; Fearon, 1994; Gibler, 2008; Tomz, 2007).
Because of this wide ranging set of costs associated with formal alliances,
alliances serve as credible signals of future intentions, both because state
leaders will only accept the costs involved in forming alliances if their intentions to cooperate are sincere and because having formed an alliance changes
incentives in favor of future cooperation. In disciplinary jargon, alliances
both entail sunk costs and tie leaders’ hands; they have both screening and
constraining effects (Fearon, 1997; Morrow, 1994; Smith, 1995).
So, how does the ability to signal future intentions credibly benefit member states? First, and most importantly, it may enhance deterrence. Formal
game-theoretic models demonstrate that adversaries are less likely to make
demands backed by threat of military force against targets with allies. All
else equal, when a challenger believes a target will receive assistance in a
war, the challenger should be more pessimistic about his probability of winning a war that might result from rejection of the demand, and thus less likely
to make the demand (Fearon, 1997; Morrow, 1994; Smith, 1995; Yuen, 2009).
Should deterrence fail, prior coordination may improve the alliance members’ ability to fight jointly, making the alliance more successful than a similar
ad hoc coalition would be (Morrow, 1994). Thus, states that feel threatened
and believe their ability to deter or fight a challenger will be enhanced by an
ally’s assistance may be motivated to seek an alliance.
Second, reliable alliances may allow states to benefit from economies of
scale in the provision of defense, thus lowering their individual defense
burdens. When economies of scale are possible, states can achieve greater
security at the same cost or the same level of security at lower cost through
cooperation (Lake, 1999, p. 7). According to this argument, therefore,
we should see alliances form when military technology is conducive to
economies of scale in the provision of defense and/or among states with
different comparative advantages in security provision.
Empirical evidence provides support for two implications of the costly signaling approach to understanding formal alliances. First, according to this
perspective, because of the costs involved in forming and violating alliances,
most alliance commitments should be sincere, and thus most alliances should
be fulfilled when they are invoked by war. When we judge reliability by
matching what state leaders did to what they promised to do in their treaties,
alliances are reliable approximately 75% of the time (Leeds, Long, & Mitchell,
2000). In addition, the best predictors of alliance violation are changes in
international power or in domestic institutions since the time of alliance formation. This suggests that leaders may have intended to fulfill alliances at

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the initial conditions, but chose to violate their past commitments after factors affecting their value for war changed (Leeds, 2003a). The fact that most
alliances are reliable when invoked by war is particularly impressive, given
the incentives of challengers to target unreliable alliances; this selection effect
most likely causes us to underestimate the true reliability of alliances in our
empirical studies (Smith, 1995).
Second, alliances should deter challengers. Leeds (2003b) and Johnson and
Leeds (2011) demonstrate that states with allies committed to assist them if
they are attacked by outside powers are less likely to be the target of militarized interstate disputes, controlling for other factors that make militarized
dispute initiation likely in a given pair of states. Benson (2011) finds that the
deterrent effect is limited to alliances that promise support to an ally conditional on the ally being attacked without having tried to alter the status
quo.
Thus, a compelling explanation for formal military alliances is that formal
agreements provide states a means to signal their future intentions credibly. Because of the costs involved in forming and maintaining alliances and
the anticipated costs of breaking a formal agreement, sincerely committed
states are more likely to be willing to form alliances. And, because peacetime military coordination improves the ability of allies to fight successfully
together and because states expect reputational costs from violating formal
agreements, members of alliances become more committed to fighting with
their allies. Thus, alliances provide reliable information to adversaries about
the probability that potential target states will receive assistance in war, discouraging challenges. Alliances also make it easier for states to be confident
that their allies will assist them in maintaining their security, allowing them
in some instances to budget less for their own defense.
CUTTING-EDGE WORK: FIVE AREAS OF CURRENT
AND FUTURE RESEARCH
Despite some significant (although not universal) agreement on the main
functions of formal alliance agreements, a number of theoretical and empirical puzzles remain. I have identified five here that contemporary scholars are
working to address.
UNDER WHAT CONDITIONS DO STATES FORM ALLIANCES?
On one hand, we have very strong theory about the conditions under which
states should form alliances and with whom they should seek to ally. States
should seek alliances when they feel threatened and expect improved deterrence and/or war-fighting ability with allied assistance. A state should seek

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an ally who is strong enough to deter or defeat the threat, shares enough
international interests to make pursuing a common foreign policy feasible,
and is likely to fulfill the commitment, should the alliance be invoked. On
the other hand, large n empirical studies have not always provided consistent support for these relationships, in part because of difficulties in research
design.
The first challenge is that alliance formation involves the simultaneous
decision of at least two states, and in the case of multilateral alliances, many
more. While the theory above tells us when states should seek alliances, we
only observe the formation of alliances when a state is successful at finding
a willing ally who also meets the criteria the state seeks. Some of the factors
that may make states most interested in finding allies (for instance, the fact
that they face significant international threats that they cannot deter alone
or that they might like to cut their defense spending) might also make other
states wary of allying with them. Similarly, some factors that make states
particularly desirable partners (for instance, high costs of violating past
commitments) may also make states more wary of making commitments
(Leeds, 1999). Thus, we need clearer theories of the dyadic/multilateral
conditions that make it more likely that we observe alliance formation.
The second challenge is that the relevant game-theoretic models feature
an identified potential challenger in addition to two potential allies. Many
of the relevant variables—power, threat, shared interests—are understandable only in the context of considering all three actors. Whether an ally is
sufficiently powerful to deter, for instance, depends on the relative power
of the potential challenger and potential target. Most existing large n studies of alliance formation have attempted to capture the external threat faced
by potential alliance members by looking at things such as the number of
militarized disputes the state has engaged in recently (e.g., Gibler & Wolford, 2006), rather than identifying a source of threat explicitly (c.f. Johnson,
2012). Of course, it is not easy to identify potential threats; more creativity and
careful thinking about how we can best operationalize threat would be very
helpful.
The result is that we do not have a great record of empirical evidence about
alliance formation that consistently supports the theory of alliances as costly
signals, because research designs have not been employed that explicitly test
the theory (see Leeds & Morgan, 2012, pp. 140–141 for a review of evidence
about specific variables). There is significant evidence about other aspects of
the theory, for instance, alliance reliability and deterrence, but it would be
useful to have tests of alliance formation that are able to better capture the
conditional relationships between power and threat, and between the attractiveness of an ally and the desire to ally.

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WHY DO STRONG STATES ALLY WITH WEAK STATES?
Alliances are intended to improve the security of states. By promising to
fight together, states aggregate their capabilities and deter challengers from
attacking them. This would suggest that stronger allies are more valuable
than weaker allies. And yet, we frequently witness alliances between strong
states and weak states, often referred to as “asymmetric” alliances. While it
makes sense that weaker states benefit from the security they gain from the
assistance of a stronger ally, some have questioned what motivates stronger
states to ally with weaker ones.
Morrow (1991) suggests that weak states who desire security from a
stronger ally may be willing to offer other concessions—for instance, support of an ally’s foreign policy—in return for an alliance. In other words,
these alliances result from issue linkage, with weaker states offering other
forms of support in return for military assistance (see also Palmer & Morgan,
2006). Fordham (2010) argues instead that because of trade relationships
and other economic interests, strong states may have a self-interest in the
security of smaller allies; defense of certain weaker states is defense of the
large state’s own economy.
Issue linkage arguments are theoretically compelling, and recent studies
have shown that issue linkage both facilitates successful negotiation of
alliance agreements and increases their credibility and compliance rates
(Poast, 2012, 2013). The security/autonomy trade-off model is quite appealing to international relations scholars, and empirical studies about duration
of alliances provide support for the theory (Morrow, 1991). That being said,
it would be nice to see empirical studies that can isolate systematic foreign
policy change in weaker states as a result of alliance formation. If indeed
strong states are compensated for providing security through compromise
on other issue areas, we should be able to document this empirically.
WHAT IS THE (FULL) RELATIONSHIP BETWEEN ALLIANCES AND WAR?
While the relationship between alliances and militarized conflict is one of the
core questions that scholars of alliance politics have addressed, there remains
some disagreement about the overall effect of alliances on conflict, mainly
because alliances change the calculations of multiple actors. Smith (1995)
argues that while alliances may deter adversaries, they may also embolden
partners, which could make the relationship between alliances and war less
clear; while challengers may be less likely to make demands, thus reducing the probability of war, if challengers do make demands, targets are more
likely to resist those demands (expecting allied support), and thus, the probability of war given a challenger demand may increase. Yuen (2009), however,
claims that because challengers make lower demands of targets with allies,

Why Do States Sign Alliances?

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it may not be the case that targets with allies are more likely to resist; they
may simply concede smaller demands. Johnson and Leeds (2011) provide an
empirical test of the probability that a state resists when targeted in a militarized dispute and find that targets with allies committed to defend them are
actually less likely to resist and escalate disputes.
Fang, Johnson, and Leeds (2014) argue that this is not only because challengers often make smaller demands of targets with allies as Yuen expected,
but also because allies sometimes act to restrain their partners from escalating disputes, encouraging them to concede demands rather than risk war
(see also Pressman, 2008; Snyder, 1997). Allies do not always wish to restrain
their partners, nor are they always successful at it; there are cases in which
targets and their allies fight the challenger together and cases in which targets choose to fight the challenger even without allied support. Fang et al. do
find, however, that targets that value their alliances highly are less likely to
escalate disputes.
Even if alliances deter challenges against member states and do not encourage member states to escalate disputes if they are targeted, alliances may
still encourage belligerent behavior on the part of member states by causing them to initiate disputes. Palmer and Morgan (2006), for example, argue
that when a state succeeds in providing for its security more efficiently, perhaps by enlisting the help of an ally, the state can use its military resources to
initiate other conflicts. They find empirical support for the claim that states
that have recently formed new alliances are more likely to initiate militarized interstate disputes. Johnson and Leeds (2011), however, find that states
with defensive alliances are not more likely to initiate militarized interstate
disputes.
It is possible that whether alliances lead to dispute initiation depends on
the prior existence of a desire to change the status quo. Benson, Bentley,
and Ray (2012) find that alliance commitments aimed at deterring changes
to the status quo are effective at reducing the probability of militarized dispute initiation among nonrevisionist states, but not among revisionist states;
revisionist states may be slightly more likely to initiate disputes with alliance
commitments, especially unconditional ones. Senese and Vasquez argue that
among states who are already engaged in a contentious relationship, particularly over territorial issues, alliances can serve as one of many “power
politics” moves that can increase threat perception and increase the likelihood that war occurs (Senese & Vasquez, 2008). Their analysis suggests that
a long-term history of outside alliances in a dyad with a territorial dispute
is associated with a higher probability of war at some point in the dyad’s
history.
We have more to untangle about the complicated effects of alliances on
international conflict. My assessment of the current state of knowledge is

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that there is more evidence that defensive alliances lower the probability of
disputes and wars than that defensive alliances cause an increase in either
disputes or wars. That being said, the relationships are quite nuanced with
a lot of moving parts, and more research about the conditions under which
alliances deter, embolden, entrap, restrain, and expand conflict is needed.
Because of the complicated strategic dynamics, game theory is well suited
to exploring these issues, but hypotheses derived from games must also be
subject to careful empirical evaluation.
HOW DO ALLIANCES AFFECT RELATIONS AMONG MEMBERS?
Most scholars and policy-makers think of alliances as primarily aimed at
affecting relations between member states and outside states, for example,
through deterrence. But, scholars have also been interested in how alliances
affect relations among their members. For instance, can alliances help to manage conflict among member states and “spillover” to encourage other cooperative relations? Several scholars have studied the function of alliances as
conflict management institutions and also their effects on other cooperative
interactions, especially international trade.
Schroeder (1976) famously referred to alliances as “tools for management,”
claiming that more powerful states might be motivated to form alliances to
“manage” the policies of other states. Similarly, Snyder (1997) and Pressman
(2008) suggest that major powers may want to restrain the foreign policy
actions of weaker powers through alliance constraints; these theories relate
closely to the issue linkage arguments regarding asymmetric alliances discussed above. Others, however, suggest that alliances may serve a general
function of making relations among member states less vulnerable to militarized conflict. Weitsman (2004) argues, for example, that alliances provide
increased transparency and a means for regular communication and thus
can be used to manage potentially adversarial relationships. Bearce, Flanagan, and Floros (2006) also argue that because alliances improve information flow among states, sharing an alliance should make militarized conflict
among states less likely, and they find support for their hypothesis in a large
n study. Long, Nordstrom, and Baek (2007) find that alliances with particular provisions for conflict management are particularly effective at reducing the probability of military conflict among members, and Gibler (1997)
finds that alliances that resolve territorial disputes lead to less conflict among
rivals.
Not only might alliances lead to less militarized conflict among member
states, but they might “spillover” to cause more cooperation in other areas
as well. Several scholars have argued, for example, that alliances lead to
increased trade among members, both because government trade policies

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favor allies and because firms seek business ties with states that they believe
are less likely to engage in conflict with their home state; this relationship has
been particularly evident since World War II (e.g., Gowa & Mansfield, 1993;
Long, 2003). Similarly, Li and Vashchilko (2010) find that alliances between
high-income and low-income states result in higher levels of bilateral foreign
investment.
In the post–Cold War era, there has been an increased tendency for
lower commitment alliance promises such as consultation and neutrality
to be embedded in broader international cooperation agreements (Leeds
& Mattes, 2007). It will be useful in the future for scholars to continue to
study not only how and why different issues get linked in an individual
treaty, but also what kinds of cooperation are most likely to pave the way for
military alliances, and what kinds of cooperation are likely to be facilitated
by military alliances.
WHAT EXPLAINS VARIANCE IN ALLIANCE DESIGN?
Finally, over the past 10 years or so, scholars have begun to pay increased
attention to variance in how alliance agreements are designed. This has been
made much easier by the existence of the Alliance Treaty Obligations and
Provisions (ATOP) dataset, which provides detailed information about the
content of 648 alliance treaties signed between 1815 and 2003 (Leeds et al.,
2002, Leeds & Mattes, 2007). Increased attention to the design of alliances
corresponds with a broader interest in the rational design of international
institutions within the discipline (Koremenos, Lipson, & Snidal, 2001).
So far, scholars have been most interested in studying the effects of variance in obligations that state leaders include in their treaties (e.g., Benson,
2011, 2012; Leeds, 2003b; Long et al., 2007). This is a crucial first step, since
an assumption of the rational design of institutions program is that leaders design agreements, expecting them to have different effects on behavior. Once we show the different effects that differently designed agreements
have, it is important also to explain the conditions under which state leaders choose different designs and why they do so. In a few recent studies,
design features have been the dependent variable (e.g., Benson, 2012; Kim,
2011; Mattes, 2012a, 2012b; Poast, 2012), but we have much more to learn. For
example, scholars can work to explain variance in provisions for managing
the alliance, for instance, in levels and types of peacetime military cooperation (e.g., Leeds & Anac, 2005), means of dispute resolution among allies,
and requirements for renegotiation. I am pleased to see research moving in
this direction because it may allow us to disentangle more clearly screening
and constraining effects of agreements, a major puzzle for all international
cooperation scholars.

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CONCLUSION
There is substantial agreement that states sign formal alliance agreements
as a costly signal of their intentions to cooperate in the event of future
military crises. Creating credible promises to cooperate has three main
benefits: increased ability to deter challenges, improved ability to win wars
should they occur, and opportunity to benefit from economies of scale in the
provision of defense. While states do sometimes violate alliance commitments in times of war and terminate them opportunistically, particularly if
significant factors affecting decisions for war have changed since the alliance
was formed, the majority of the time alliances are reliable. Even within the
“high politics” arena of national security, there is efficacy in international
cooperation.
There has been significant progress in this research area over the last
two decades. The development of game-theoretic models that are able to
capture strategic interaction between allies and adversaries has significantly
advanced our theoretical understanding of why states sign formal alliances
and how these formal agreements affect behavior. The collection of new
data that provides additional information about the content of formal
alliance agreements, combined with clever research designs, has allowed
for targeted empirical tests of several hypotheses drawn from these models.
And yet, while we know a lot, several important puzzles remain. The
current generation of scholars should continue to analyze, both theoretically
and empirically, how states design their cooperative agreements to achieve
their goals and the intended and unintended effects of these cooperative agreements on a variety of behaviors of member and nonmember
states.
ACKNOWLEDGMENTS
The author would like to thank Jesse Johnson, Michaela Mattes, and Paul
Poast for comments and suggestions, and Naoko Matsumura and Ahra Wu
for research assistance.

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FURTHER READING
Leeds, B. A. (2003a). Alliance reliability in times of war: Explaining state decisions to violate treaties. International Organization, 57, 801–827. doi:10.1017/
S0020818303574057
Leeds, B. A. (2003b). Do alliances deter aggression? The influence of military alliances
on the initiation of militarized interstate disputes. American Journal of Political Science, 47, 427–439. doi:10.1111/1540-5907.00031
Morrow, J. D. (2000). Alliances: Why write them down? Annual Review of Political
Science, 3, 63–83. doi:10.1146/annurev.polisci.3.1.63
Morrow, J. D. (1994). Alliances, credibility, and peacetime costs. Journal of Conflict
Resolution, 38, 270–297. doi:10.1177/0022002794038002005
Snyder, G. H. (1997). Alliance politics. Ithaca, NY: Cornell University Press.

BRETT ASHLEY LEEDS SHORT BIOGRAPHY
Brett Ashley Leeds is Professor of Political Science at Rice University. (link
to webpage: http://www.ruf.rice.edu/∼leeds/) Her research focuses on the
design and influence of international agreements, compliance with international commitments, and the influence of domestic politics on international
relations. She has written extensively on the politics of military alliances. Her
recent articles have appeared in American Journal of Political Science, Journal of

14

EMERGING TRENDS IN THE SOCIAL AND BEHAVIORAL SCIENCES

Politics, International Organization, Foreign Policy Analysis, and Journal of Peace
Research. In 2008, Leeds was awarded the Karl Deutsch Award by the International Studies Association, which is presented annually to a scholar in IR
under age 40 or within 10 years of defending his or her dissertation who is
judged to have made, through a body of publications, the most significant
contribution to the study of International Relations and Peace Research.
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