Alliance, in international politics, an association of two or more nations united by a formal treaty for some agreed 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 formation of a system of alliances between non-Communist countries around the world, for example, the North Atlantic Treaty Organization. The Warsaw Pact, formed by European Communist countries, was also formally an alliance. An alliance may sometimes be made to achieve economic goals, for example, the Alliance for Progress.