Of all human activities, war is the least rational. It costs a fortune. It spreads death and misery, from the killing fields of Sudan to the tunnels of Gaza. It is often started out of personal hubris or blind patriotic zeal: think of Napoleon’s invasion of Russia or Japan’s decision in 1941 to provoke a war with a superpower it could not hope to defeat. So you might think economics—a discipline associated with rational self-interest—would have little to say about it. You would be wrong, argues Duncan Weldon, a former writer for and occasional contributor to The Economist, in “Blood and Treasure”.