What causes war? Thucydides thinks fear, honor, and interest—those fundamental human motivations that persuade us beyond caution—cause wars. Clausewitz tries to leach those passions out of the process and distill a calculus of political aims as the cause of war. Geoffrey Blainey has a simpler discriminator: states choose war when they think they will win. Barbara Tuchman has the simplest explanation of all: human folly. Azar Gat believes scarcity drives warfare, and, therefore, prosperity is making it obsolete. The author analyzes the contributions of these five writers in addressing this perennial question about war.