Walter Russell Mead acknowledges in God and Gold: Britain, America, and the Making of the Modern World (Vintage) that balance of power politics is a matter of letting rivals busy their giddy minds with foreign quarrels. Britain was happy to leave Continental fights to Continentals: “Let France and Prussia duke it out on the Rhine; let Austria and Prussia batter one another blood over Silesia, an irregular, slightly sausage-shaped territory now part of Poland that is roughly equal to the combined area of Connecticut and Massachusetts.” While they were doing that, the Brits were busy building “a global system that would leave all rivals in the dust.”

Part of the trick, though, is intervening to keep the balance in balance, and that means helping out the weaker side. The British became masters of the technique after the Treaty of Utrecht ended the War of the Spanish Succession (1713), but the Americans converted it into a global strategy: “Today, for example, the United States continues to work toward the emergence of a stable balance of power in Asia, building new partnerships with old enemies like Vietnam. During the Cold War, Washington saw the USSR as the great threat to the balance of power in Asia, and lined up all the allies it could to balance it, including both China and Japan in its coalition.” Weak is relative, and so balance of power politics has to be fluid: “From World War I through 1949 the United States took a different tack following the same balance-of-power logic and sided with China against Japan.”