When a Serbian-backed assassin gunned down Archduke Franz Ferdinand in late June 1914, the world seemed unmoved. Even Ferdinand’s own uncle, Franz Josef I, was notably ambivalent about the death of the Hapsburg heir, saying simply, “It is God’s will.” Certainly, there was nothing to suggest that the episode would lead to conflict—much less a world war of such massive and horrific proportions that it would fundamentally...