The kings, generals, and prime ministers who controlled europe's armies in the summer of 1914 didn't think their aggressive behaviors-issuing ultimatums, calling up reserves, massing troops on one another's borders-would result in war. Rather, they believed that conspicuous muscle flexing would impel their rivals to back down. But each show of force on one side prompted an even more extravagant riposte, until the march to war became unstoppable-and so tens of millions perished.