From InfoPlease.com: Balance of Power:
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, but this ended with the collapse of the USSR in 1991.
See H. J. Morgenthau, Politics Among Nations (1960); H. Butterfield and M. Wright, ed., Diplomatic Investigations (1966); P. Keal, Unspoken Rules and Superpower Dominance (1984); R. J. Lieber, No Common Power: Understanding International Relations (1988).