Military alliance

A military alliance is a formal agreement between nations that specifies mutual obligations regarding national security. In the event a nation is attacked, members of the alliance are often obligated to come to their defense regardless if attacked directly. Military alliances can be classified into defense pacts, non-aggression pacts, and ententes. Alliances may be covert (as was common from 1870 to 1916) or public.

According to a 2002 dataset of military alliances, there have been 538 alliance treaties from 1815 to 2003. The vast majority of the alliances involve commitments to come to the military support of one ally involved in war. The vast majority are defensive in nature. Since the end of the Second World War, military alliances have usually behaved less aggressively and act more as a deterrent. A 2025 analysis of alliances across the 19th and 20th century found that alliances overall tend to deter war.

Military alliances are related to collective security systems but can differ in nature. An early 1950s memorandum from the United States Department of State explained the difference by noting that historically, alliances "were designed to advance the respective nationalistic interests of the parties, and provided for joint military action if one of the parties in pursuit of such objectives became involved in war." A collective security arrangement "is directed against no one; it is directed solely against aggression. It seeks not to influence any shifting 'balance of power' but to strengthen the 'balance of principle'."

The obvious motivation in states engaging in military alliances is to protect themselves against threats from other countries. However, states have also entered into alliances to improve ties with a particular nation or to manage conflict with a particular nation.

The nature of alliances, including their formation and cohesiveness (or lack thereof), is a subject of much academic study past and present. Influential works include those by Glenn Snyder, Stephen Walt, and Kenneth Waltz. Kenneth Waltz outlined in his 1979 book Theory of International Politics a neorealist theory of international politics where he argued that balances of power tend to form in world politics. Alongside neoliberalism, neorealism is one of the two most influential contemporary approaches to the study of military alliances in international relations; the two perspectives dominated international relations theory from the 1960s to the 1990s. Neorealism emerged from the North American discipline of political science, and reformulates the understanding of military alliances in the classical realist tradition of E. H. Carr, Hans Morgenthau, George Kennan and Reinhold Niebuhr. Neorealism is subdivided into defensive and offensive neorealism.

During peace-time, according to a 2019 study, almost all alliances from 1870 to 1916 may have been covert or implied. In other time periods, covert alliances have been rare. The study argues that from 1870 to 1916, the unusual amount of covert alliances was incentivized by other covert alliances. The creation of public alliances would signal to the covert ally that the public alliance was more valuable. According to Ronald Krebs, pre-WWII alliances were generally "relatively simple, short-lived affairs."

According to a network study by Jackson and Nei (2015), the history of interstate alliances since the early nineteenth century can be divided roughly into a thin and unstable pre-1950 era and a more than four-times denser and more stable network after 1950. That later period with a denser and more stable network of alliances coincides with a sharp decline in interstate wars and an increase in trading relationships. The same analysis reports that a larger number of allies correlates with being attacked less frequently, and that greater bilateral trade correlates with less conflict.

Common problems for alliances revolve around free-riding and burden-sharing. Members of an alliance have incentives not to contribute to the alliance while simultaneously benefiting on the public goods provided by the alliance. According to Mancur Olson and Richard Zeckhauser's classic study of alliances, small states frequently free-ride on the large state's contributions to an alliance. Small allies that are militarily vulnerable are less likely to free-ride, whereas strategically important small allies are most likely to free-ride. Alliances may also lead to moral hazard whereby allies behave more aggressively and recklessly if they believe that the alliance will aid them in any conflict. On the whole, alliances do deter aggression on net.