To be explained are the Great Power versus Great Power wars, 1816–1989. Familiar strands of ‘realist’ thinking are twisted into a good simple balances of power theory to explain peace, war and the extent of war. The evidence, quantitative and qualitative, is that non-separated approximate equals in power capabilities formed dangerous balances. Generally, would-be-combatant great powers used alliances to restrain stronger third parties and to add the capabilities of weaker states to their own. The critical importance of political geography, a state's political capacity, and alliance commitments to the balance of power are made plain.