balance of power, system of international relations in which nations seek to maintain an approximate equilibrium of power among many rivals, thus preventing the preponderance of any one state. Crucial to the system is a willingness on the part of individual national governments to change alliances as the situation demands in order to maintain the balance. Thucydides' description of Greece in the 5th cent. B.C. and Guicciardini's description of 15th-century Italy are early illustrations. Its modern development began in the mid-17th cent., when it was directed against the France of Louis XIV. Balance of power was the stated British objective for much of the 18th and 19th cent., and it characterized the European international system, for example, from 1815-1914. After World War I the balance of power system was attacked by proponents of cooperation and a community of power. International relations were changed radically after World War II by the predominance of two superpowers, the United States and the Soviet Union, with major ideological differences between them. After the 1960s, with the emergence of China and the Third World, a revived Europe and Japan, it reemerged as a component of international relations. With the collapse of the USSR in 1991, the United States, as the sole remaining superpower, has been dominant militarily and, to a lesser degree, economically.

In international relations, an equilibrium of power sufficient to discourage or prevent one nation or party from imposing its will on or interfering with the interests of another. The term came into use at the end of the Napoleonic Wars to denote the power relationships in the European state system. Until World War I, Britain played the role of balancer in a number of shifting alliances. After World War II, a Northern Hemisphere balance of power pitted the U.S. and its allies (see NATO) against the Soviet Union and its satellites (see Warsaw Pact) in a bipolar balance of power backed by the threat of nuclear war. China's defection from the Soviet camp to a nonaligned but covertly anti-Soviet stance produced a third node of power. With the Soviet Union's collapse (1991), the U.S. and its NATO allies were recognized universally as the world's paramount military power.

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