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. And with the headlines from Ukraine getting scarier by the day, this seems like a good time to ask why.

Once upon a time wars were fought for fun and profit; when Rome overran Asia Minor or Spain conquered Peru, it was all about the gold and silver. And that kind of thing still happens. 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. War in the preindustrial world was and still is more like a contest among crime families over who gets to control the rackets than a fight over principles.

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. Furthermore, it’s very hard to extract golden eggs from sophisticated economies without killing the goose in the process.

We might add that modern war is very, very expensive. For example, by any estimate the eventual costs (including things like veterans’ care) of the Iraq war will end up being well over $1 trillion, that is, many times Iraq’s entire G.D.P.