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Alliances are among the most studied aspects of international cooperation and play important roles in most broad theories of international relations, particularly theories of war and peace. They are an important component of the security policies of most states in the international system today. This entry first defines and describes alliances, then discusses the motivations of states to form alliances and the effects of alliances on international outcomes.

An alliance is a formal agreement among independent states in the international system to cooperate militarily in the event of militarized conflict with outside parties. Alliances are distinguished from informal alignments by their codification in a written document; while states that share interests and tend to coordinate behavior may be termed aligned, only states that have made an international legal commitment to assist one another are allied. Alliances are distinguished from protectorates, unilateral guarantees, and colonial relationships by their joint commitments; all member states retain sovereignty and independence but commit to adjust their policies to meet allied goals. Commitments need not be symmetrical, but all alliance members must make some cooperative commitment. Finally, alliances involve promises of assistance in the event of military conflict with outside parties. Thus, they are distinguishable from other forms of military cooperation such as arms sales agreements and intelligence-sharing agreements.

Formation and Provisions of Alliances

Most alliances are formed through treaties. The specific promises made in these treaties can vary significantly. Some alliances commit the signatories to assist one another in the event a member state is attacked; these are often referred to as defense pacts. An example of a defense pact is the North Atlantic Treaty Organization (NATO). Other alliances provide for states to assist one another in accomplishing offensive goals, usually in addition to defensive promises. The Pact of Steel, signed by Germany and Italy in 1939, is an example of this sort of alliance. Many alliances, however, fall short of guaranteeing active participation in conflicts that may arise and instead commit the member states to remain neutral and provide no support for the adversary of an ally in the event the ally becomes involved in conflict or commit the members to consult in the event of threat and make every attempt to produce a coordinated response, without any specificity about what that coordinated response might be. The Molotov-Ribbentrop Pact (also known as the Nazi-Soviet Nonaggression Pact) signed in 1939 is an example of a neutrality pact, and the Treaty of Good Neighborly Friendship and Cooperation signed by Russia and China in 2001 is an example of a consultation pact (also known as an entente).

In many cases, alliance treaties also specify the conditions under which the obligations come into force. Rather than applying to any military conflict, an alliance may, for instance, apply only to conflicts with specific adversaries, in specific locations, or in conjunction with a specific dispute. The NATO treaty, for instance, specifies that it applies only to the home territories of members and does not commit members to defend their allies' colonial possessions. Defense pacts signed by France with both Poland and Czechoslovakia at Locarno, Switzerland, in 1925 commit the states to assist one another only in the event of unprovoked attack by Germany. And some treaties (for instance, the Convention of London signed by France, the United Kingdom, and Italy during the Crimean War) are limited to a specific ongoing conflict, committing the states to work together only through the conclusion of the current war.

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