The balance of power—a notoriously slippery, murky, and protean term, endlessly debated and variously defined—is the core theory of international politics within the realist perspective. A “balance of power” system is one in which the power held and exercised by states within the system is checked and balanced by the power of others. Thus, as a nation’s power grows to the point that it menaces other powerful states, a counter-balancing coalition emerges to restrain the rising power, such that any bid for world hegemony will be self-defeating. The minimum requirements for a balance of power system include the existence of at least two or more actors of roughly equal strength, states seeking to survive and preserve their autonomy, alliance flexibility, and the ability to resort to war if need be.
At its essence, balance of power is a type of international order. Theorists disagree, however, about the normal operation of the balance of power. Structural realists describe an “automatic version” of the theory, whereby system balance is a spontaneously generated, self-regulating, and entirely unintended outcome of states pursuing their narrow self-interests. Earlier versions of balance of power were more consistent with a “semi-automatic” version of the theory, which requires a “balancer” state throwing its weight on one side of the scale or the other, depending on which is lighter, to regulate the system. The British School’s discussion of balance of power depicts a “manually operated” system, wherein the process of equilibrium is a function of human contrivance, with emphasis on the skill of diplomats and statesmen, a sense of community of nations, of shared responsibility, and a desire and need to preserve the balance of power system.
As one would expect of a theory that made its appearance in the mid-16th century, balance of power is not without its critics. Liberals claim that globalization, democratic peace, and international institutions have fundamentally transformed international relations, moving it out of the realm of power politics. Constructivists claim that balance of power theory’s focus on material forces misses the central role played by ideational factors such as norms and identities in the construction of threats and alliances. Realists, themselves, wonder why no global balance of power has materialized since the end of the Cold War.