Balance of Power, a relatively equal distribution of economic and military strength among rival countries or groups of countries. For 400 years the countries of Europe devoted much of their diplomatic and military effort to creating or maintaining such a balance. Their object was to prevent any single nation or group of nations from becoming powerful enough to dominate the continent.

As one European nation or group grew stronger, neighboring countries built up their own strength or formed alliances for mutual defense. Nations continually shifted their support as their allies or rivals changed their aims or grew stronger or weaker. The countries involved generally tried to maintain a balance through diplomacy, but when the balance was seriously disrupted, war nearly always followed.

The idea of maintaining a power equilibrium became an important influence in European politics after the rise of national states in the 16th century. An outstanding example of balance-of-power politics occurred in the Thirty Years' War (1618–48). Cardinal Richelieu, fearing the power of Austria under the Hapsburgs, sent armies of predominantly Catholic France to the aid of the Protestant nobles of Germany and Sweden. In the 18th century, England, Holland, and Austria united against Louis XIV of France when his plan for combining the French and Spanish crowns threatened the balance of power. About a century later a similar alliance ended Napoleon's ambition to dominate Europe. In the 19th century, Great Britain was a decisive influence in maintaining or restoring the balance by supporting the weaker nations. During 1854–56, for example, Britain and other European powers halted Russian expansion by aiding Turkey in the Crimean War. By the early 20th century, the balance of power in Europe had become so delicate that only a minor incident was needed to set off a major war. After World War I efforts were made to create a new balance of power through disarmament conferences and the League of Nations. These attempts failed and Germany grew to be far more powerful than its neighbors.

The term “balance of power” is rarely used today to refer to contemporary international relations except in the case of the Middle East, where a delicate balance exists between Israel and its Arab neighbors. From 1949, when the Soviet Union acquired nuclear weapons, to 1990, when the Cold War ended, the two “superpowers"—the United States and the Soviet Union—still relied on alliances, but their overall strategy was based on the concept of nuclear deterrence—of making war on each other so costly that neither nation would dare to attack the other. Nuclear deterrence is sometimes referred to as the “balance of terror."