A century has passed since the start of World War I, which many people at the time declared was the war to end all wars." Unfortunately, wars just kept happening. In influential research sponsored by the World Bank, the Oxford economist Paul Collier has shown that the best predictor of civil war, which is all too common in poor countries, is the availability of lootable resources like diamonds. Whatever other reasons rebels cite for their actions seem to be mainly after-the-fact rationalizations. If you're a modern, wealthy nation, however, war - even easy, victorious war - doesn't pay. And this has been true for a long time. In his famous 1910 book The Great Illusion, the British journalist Norman Angell argued that "military power is socially and economically futile." As he pointed out, in an interdependent world (which already existed in the age of steamships, railroads, and the telegraph), war would necessarily inflict severe economic harm even on the victor. Modern nations can't enrich themselves by waging war. Yet wars keep happening. Why? Governments all too often gain politically from war, even if the war in question makes no sense in terms of national interests. Nations almost always rally around their leaders in times of war, no matter how foolish the war or how awful the leaders. Argentina's junta briefly became extremely popular during the Falklands war. For a time, the "war on terror" took President George W. Bush's approval to dizzying heights, and Iraq probably won him the 2004 election. True to form, Mr. Putin's approval ratings have soared since the Ukraine crisis began.