Alliance (politics), in international politics, an association of two or more nations united by a formal treaty for some agreed-upon purpose. Most alliances are defensive in form, involving a pledge of mutual military assistance against an actual or potential common enemy. Alliances have existed throughout history, from ancient China and India to the modern nations, often as responses to acts of aggression. The threat of Soviet expansion after World War II caused the United States to change its traditional policy of nonalignment. It now maintains a system of alliances with non-Communist countries around the world, for example, the North Atlantic Treaty Organization. An alliance may sometimes be made to achieve economic goals, for example, the Alliance for Progress.

See also Confederation; Holy Alliance; Treaty; Triple Alliance; Triple Entente.