Balance Of Power, a phrase in international law for such a "just equilibrium" between the members of the family of nations as should prevent any one of them from becoming sufficiently strong to enforce its will upon the rest. The principle involved in this, as Hume pointed out in his Essay on the Balance of Power, is as old as history, and was perfectly familiar to the ancients both as political theorists and as practical statesmen. In its essence it is no more than a precept of commonsense born of experience and the instinct of self-preservation; for, as Polybius very clearly puts it (lib. i. cap. 83): "Nor is such a principle to be despised, nor should so great a power be allowed to any one as to make it impossible for you afterwards to dispute with him on equal terms concerning your manifest rights." It was not, however, till the beginning of the 17th century, when the science of international law took shape at the hands of Grotius and his successors, that the theory of the balance of power was formulated as a fundamental principle of diplomacy.
According to this the European states formed a sort of federal community, the fundamental condition of which was the preservation of the balance of power, i.e. such a disposition of things that no one state or potentate should be able absolutely to predominate and prescribe laws to the rest; and, since all were equally interested in this settlement, it was held to be the interest, the right and the duty of every power to interfere, even by force of arms, when any of the conditions of this settlement were infringed or assailed by any other member of the community. This principle, once formulated, became an axiom of political science. It was impressed as such by Fénelon, in his Instructions, on the young duke of Burgundy; it was proclaimed to the world by Frederick the Great in his Anti-Machiavel; it was re-stated with admirable clearness in 1806 by Friedrich von Gentz in his Fragments on the Balance of Power. It formed the basis of the coalitions against Louis XIV. and Napoleon, and the occasion, or the excuse, for most of the wars which desolated Europe between the congress of Münster in 1648 and that of Vienna in 1814. During the greater part of the 19th century it was obscured by the series of national upheavals which have remodelled the map of Europe; yet it underlay all the efforts of diplomacy to stay or to direct the elemental forces let loose by the Revolution, and with the restoration of comparative calm it has once more emerged as the motive for the various political alliances of which the ostensible object is the preservation of peace (see Europe: History).
An equilibrium between the various powers which form the family of nations is, in fact, - as Professor L. Oppenheim (Internat. Law, i. 73) justly points out - essential to the very existence of any international law. In the absence of any central authority, the only sanction behind the code of rules established by custom or defined in treaties, known as "international law," is the capacity of the powers to hold each other in check. Were this to fail, nothing could prevent any state sufficiently powerful from ignoring the law and acting solely according to its convenience and its interests.
See, besides the works quoted in the article, the standard books on International Law (q.v.).
(W. A. P.)
 Emerich de Vattel, Le Droit des gens (Leiden, 1758).