Hegemonic Threats and Great-power Balancing

The central proposition of balance-of-power theory (albeit one that has never been tested systematically) is that great powers balance against hegemonic threats. This article argues that this proposition applies to hegemonic concentrations of land-based military power in autonomous continental systems, but not necessarily to hegemonic concentrations of sea power in maritime systems. With a focus on continental systems, this article develops and tests several hypotheses linking military concentration, capability changes, and alliance responses for the European system from 1495 to 1999. Judging from existing data on army concentrations and a new database of great-power alliances since 1495, European great powers have demonstrated a strong propensity to balance when one state has acquired a third or more of the total military capabilities in the system, but not at lower concentrations of power; higher concentrations of power usually lead to larger balancing coalitions. Great powers do not always balance, however, so balancing is a probabilistic tendency rather than an “iron law” of behavior.