World War I (1917): US declaration

The 1914-1918 industrial war that broke empires, enfranchised mass mourning, and left a peace that did not hold. The First World War began with Austria-Hungary's declaration of war on Serbia on 28 July 1914, after the assassination of Archduke Franz Ferdinand in Sarajevo on 28 June. Within a week the alliance system had pulled in Germany, Russia, France and Britain; the Ottoman Empire and later Italy, Bulgaria and the United States widened the map.

About 15-22 million people died, scholarly range, including roughly 9-11 million military dead and a large civilian toll from blockade, massacre and the 1918 influenza that overlapped the last campaigns. The Western Front's trenches dominate English memory; the Eastern Front, the Alps, Mesopotamia, East Africa and the sea war were equally part of the same industrial conflict.

Four empires -- German, Austro-Hungarian, Ottoman and Russian -- collapsed. The 1919-23 peace settlements created new states and mandates, and also the grievances that Hitler and others later exploited. Poppies, cenotaphs and the two-minute silence remain the Anglophone template for remembering war itself.