Military alliance

European military alliances prior to World War ITriple Entente and Triple Alliance
Two military alliances (NATO and Warsaw Pact) in Europe during the Cold War

A military alliance is an international agreement concerning national security in which the contracting parties agree to mutual protection and support in case of a crisis that has not been identified in advance.[1] Military alliances differ from coalitions, which formed for a crisis that already exists.[1]

Military alliances can be classified into defense pacts, non-aggression pacts , and ententes.[2] Alliances may be covert (as was common from 1870 to 1916) or may be public.[3]


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.'"[4]

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.[5]

The nature of alliances, including their formation and cohesiveness (or lack thereof), is a subject of much academic study past and present, with the leading scholars generally considered to be Glenn Snyder and Stephen Walt.[6]

According to a 2019 stidy, almost all alliances from 1870 to 1916 were covert. 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.[3]