As President Joe Biden hosted the July 2024 NATO 75th anniversary summit, the alliance leaders who attended likely had another president on their mind. Former president and GOP candidate Donald Trump raised concerns with a complaint on Truth Social that NATO allies had been “delinquent” – and only “paid up” under his watch. While remarks like this from Trump and others aren’t new, these comments put U.S. defense alliances in the global spotlight again.
The United States is currently a member of several defense pacts, the most notable being the North Atlantic Treaty Organization (NATO). In recent years, Trump and others have questioned whether these commitments are valuable to the U.S. Let’s look at these questions in depth – here’s why countries form alliances, and how these alliances affect the likelihood of war.
What is an alliance?
While policymakers and pundits use the term “ally” to refer to friendly countries in general, this discussion will focus on formal military alliances. These are the written legal agreements that include promises by one country to assist another in the event of military conflict.
More specifically, this explainer will focus on a subset of alliances that obligate members to defend their partners from attacks on their sovereignty or territorial integrity, using military force if necessary. Political scientists refer to these types of agreements as defense pacts. Currently the U.S. has defense pacts with most of Europe, through NATO. The U.S. also has defense pacts with most of Latin America through the Organization of American States (OAS). And the U.S. has defense pacts with countries like Australia, Japan, South Korea, the Philippines, Pakistan, and Thailand.
Other types of defense arrangements, like defense cooperation agreements, are beyond the scope of this explainer. It’s important to remember that a defense pact is a very specific promise to use military force to come to the aid of the other member(s) of the pact. The United States does not have such a defense pact with many countries that it counts as friends, including Israel, Saudi Arabia, and Taiwan.
Why do countries seek defense alliances?
Countries form defense pacts in order to deter threats from adversaries. Promising to work together in the face of conflict creates a stronger joint fighting force and also allows for the development of economies of scale in the provision of defense. For example, if other countries can depend on the U.S. nuclear deterrent, they don’t need to develop their own nuclear forces.
Formalizing alliances is important for two reasons. First, it allows for coordination among the allies. Joint military bases, along with programs for peacetime military coordination and troop interoperability, make fighting jointly in the future easier. This type of integration also increases incentives for members to fulfill their alliance commitments in the event of conflict. Second, making public legal commitments means a country invests its reputation in the relationship. Investing both military capacity and reputation in the relationship communicates the shared interests of the allied countries to potential adversaries.
Why does the U.S. need alliances? Or does it?
The U.S. is the strongest military power in the world, with the largest defense budget, most advanced technology, and highest gross domestic product. The U.S. geographic location with few immediate neighbors aids in its defense. One might wonder, then, why forming alliances with weaker powers is beneficial to the United States. No one expects South Korea, for example, to be decisive in responding to a military threat against America.
However, stronger countries see several reasons to guarantee the defense of other countries. First, the U.S. has worldwide economic and political interests. Conflict that does not involve the U.S. can be quite costly to U.S. firms and to the stability of the world. Second, weaker countries often offer concessions to stronger allies in return for protection. These protections may provide economic advantages, diplomatic support, or the use of their territory for military bases, for example. Third, alliances give the U.S. a prominent seat at the bargaining table as rivals of the U.S. negotiate with U.S. partners, allowing the U.S. to boost its global influence.
Are these alliances a tool for war, or peace?
A final point to consider is that defense pacts make it less likely that a country will be the target of military aggression. My research, for instance, finds evidence that defense pacts can deter attacks on their members. This is a key reason that Ukraine seeks NATO membership.
Some worry that having a strong alliance to back them up, via a defense pact, may make members behave more aggressively in the international system. In fact, the evidence for this phenomenon is at best mixed. It’s not clear that alliances encourage countries to behave more aggressively. After all, allies are in a position to deny support to partners who make unreasonable demands and promise support when adversaries make unreasonable demands. That makes allies particularly effective conflict managers, with the ability to impose their preferred bargain on both sides.
Defense pacts make war less likely, but they do not eliminate war entirely. And past history reminds us that when wars break out among countries with allies committed to defend them, these wars can expand to become broader conflagrations. While the U.S. could certainly choose to abandon its allies and not join in future wars, the U.S. government would face significant domestic and international pressure to fulfill its commitments. Thus, if Ukraine were to become a member of NATO, future attacks on Ukraine would be less likely. But should an attack occur, there would be even more pressure for the U.S. and other NATO allies to join the conflict.
Defense pacts make it more likely that if war occurs involving a U.S. ally, the U.S. will join in that war. But defense pacts also protect the global status quo by reducing the probability of war occurring in the first place. And should conflicts arise, defense pacts help secure an influential place for the U.S. at the bargaining table.
If the U.S. values stability in the international system, maintaining credible alliance commitments helps to achieve that goal through the influence these long-standing commitments give the U.S. over the behavior of both adversaries and allies.
Brett Ashley Leeds is Radoslav Tsanoff Professor of Political Science at Rice University. As part of her research on military alliances, Leeds maintains the Alliance Treaty Obligations and Provisions (ATOP) dataset.
Related Good Authority posts
- Stacie Goddard, “Good to Know: Collective security.” From January 2024, an explainer on the NATO alliance.
- Stacie Goddard, “After the explosion in Poland, will NATO go to war with Russia? Probably not.” From November 15, 2022, after an errant missile (ultimately determined to be from Ukraine’s air defense system) landed in Poland.
- Caitlin Talmage, “The Ukraine crisis is now a nuclear crisis.” From February 27, 2023, when Russian President Vladimir Putin put Russia’s nuclear forces on alert.
- Sara Moller, “NATO can’t send troops to Ukraine. Here is what it will probably do instead.” From February 28, 2022, after the Russian invasion of Ukraine.
- Michael Allen, Michael Flynn, and Carla Martinez-Machain, “Biden will stop the U.S. troop drawdown in Germany, but also push for smaller deployments around the world.” From February 10, 2021, following President Biden’s first foreign policy speech.
- James Goldgeier, “Trump’s national security adviser wants to water down U.S. NATO commitments. Here’s what that means.” From November 20, 2016, after Trump’s election victory.
- Jordan Becker and Edmund Malesky, “Yes, NATO is sharing the defense burden. Here’s what we found.” From December 9, 2016, as U.S. allies anticipate new demands from newly elected President Donald Trump.
- See also: Good Authority topic guide on NATO and European Security.
Further reading
- Songying Fan, Jesse C. Johnson, and Brett Ashley Leeds, “To Concede or To Resist? The Restraining Effect of Military Alliances,” International Organization 2014, 68 (4): 775-809.
- Matthew Fuhrmann and Todd S. Sechser, “Signaling Alliance Commitments: Hand-Tying and Sunk Costs in Extended Nuclear Deterrence,” American Journal of Political Science 2014, 58 (4): 919-935.
- Jesse C. Johnson and Brett Ashley Leeds, “Defense Pacts: A Prescription for Peace?” Foreign Policy Analysis 2010, 7 (1): 45-65.
- Brett Ashley Leeds, “Do Alliances Deter Aggression? The Influence of Military Alliances on the Initiation of Militarized Interstate Disputes,” American Journal of Political Science 2003, 47: 427-439.
- James D. Morrow, “Alliances and Asymmetry: An Alternative to the Capability Aggregation Model of Alliances,” American Journal of Political Science 1991, 35 (4): 904–933.
- Alastair Smith, “Alliance Formation and War,” International Studies Quarterly 1995, 39 (4): 405–425.
- Michael Tomz and Jessica L.P. Weeks, “Military Alliances and Public Support for War,” International Studies Quarterly 2021, 65 (3): 811–824.
Last updated August 2024
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