Entering the First World War

Wilson won reelection in 1916 on neutrality, and for two and a half years the United States stayed out of Europe's war while trading heavily with Britain and France. The shift came from two shocks arriving almost together in early 1917. First, Germany announced it would resume unrestricted submarine warfare, sinking any ship, including American merchant and passenger vessels, approaching Allied ports. This directly threatened American lives and trade, which neutrality had never protected. Second, British intelligence intercepted the Zimmermann telegram, a German proposal urging Mexico to attack the United States if war came, promising Mexico its lost territory in Texas, New Mexico, and Arizona as a reward. Published in American newspapers in March 1917, it turned a distant European conflict into a direct threat on the continent. Congress declared war in April 1917.

American troops did not reach France in large numbers until 1918, after three years of trench deadlock had exhausted both sides' manpower and supplies. Fresh, well-equipped American divisions tipped that balance, letting the Allies push forward rather than simply hold lines. The Meuse-Argonne offensive, fought by over a million American troops from September to November 1918, pushed German forces back through the Argonne Forest but cost roughly 26,000 American dead in six weeks, the deadliest battle in American history. That offensive helped force the armistice on November 11, 1918, closing the war and opening the fight over Wilson's peace terms at Versailles.