BALANCE OF POWER, in international relations, such a "just equilibrium" between nations as shall prevent any one of them being in a position to dominate the rest. The principle involved is as old as history, and was familiar to the ancients both as theorists and practical statesmen (see e.g., Polybius, 1. i. cap. 83) . In modern Europe it was first adopted as a fundamental maxim of statecraft in Italy, and later, as the conception of the sovereign state superseded feudal principles, everywhere else. Early in the 17th century, when international law began to take shape at the hands of Grotius and his successors, the theory of the balance of power was formulated as a fundamental principle. According to this, the European states formed a sort of federation based on the balance of power, and it was the right and duty of every state to interfere, if necessary by arms, when any of the conditions of the settlement were infringed by any other member of the community (see Vattel, Le Droit des gens, 1758). This principle was generally accepted. It was the justification of the coalitions against Louis XIV. and Napoleon and the occasion, or excuse, for most of the wars waged between the date of the Congress of Westphalia in 1648 and that of Vienna in 1814.
During the greater part of the 19th century the principle was obscured by the national upheavals which changed the map of Europe; but towards the end of the century it emerged again in the series of alliances and counter-alliances of which the osten sible object was to preserve peace. The outbreak of the World War is widely held to have discredited the whole principle of the balance of power. It would be truer to say, however, that the war broke out because Great Britain's attitude up to the last mo ment was uncertain, and that the balance had therefore not been kept conspicuously even. In any case, as the late Professor Oppen heim pointed out (Internat. Law i. 73), so long as there is no central authority capable of enforcing the rules of international law, the only sanction behind them, apart from the uncertain force of public opinion, is the capacity of the powers to hold each other in check. The League of Nations (q.v.) is a tentative effort to es tablish what H. H. Asquith called "a community of power" in place of "the balance of power." It has not been able to prevent, though it has done something to check and regularize, the system of separate alliances. In the United States, it may be added, the principle of the balance of power has always been denounced. This is easy to understand ; for, in the expressive language of Secretary Olney, in the Americas "the fiat of the United States is law." The Latin Americans, equally naturally, dream of an ultimate balance.
See David Hume, Essay on the Balance of Power (1752) ; Fr. v. Gentz, Fragments on the Balance of Power (18o6), and generally the standard works on International Law (q.v.) . (W. A. P.)