Wilson campaigned on keeping America out of the war, then asked Congress to declare it. The Zimmermann Telegram — Germany offering Mexico Texas, New Mexico, and Arizona — made neutrality impossible.
President Woodrow Wilson had won the 1916 election partly on the slogan 'He kept us out of war.' Yet on April 2, 1917, he stood before a joint session of Congress and asked for a declaration of war against Germany — a decision he had long resisted and that visibly pained him. He reportedly said to an aide afterward: 'Think what it was they were applauding.'
Two events ended American neutrality. Germany announced in February 1917 that it would resume unrestricted submarine warfare — sinking any ship near Britain, including American vessels. Then the Zimmermann Telegram became public: a German diplomatic proposal for Mexico to attack the United States in exchange for the return of Texas, New Mexico, and Arizona.
The Senate voted 82 to 6 for war on April 4. The House passed it 373 to 50 on April 6, when Wilson signed the declaration. Among the dissenters was Jeannette Rankin of Montana — the first woman elected to Congress — who voted no despite knowing it would cost her re-election. It did.
Wilson framed American entry as a moral crusade: the world must be made safe for democracy. His Fourteen Points, announced in January 1918, outlined a vision for post-war order based on self-determination, open diplomacy, and a League of Nations — ideas that would shape, and ultimately frustrate, the Paris Peace Conference.
American entry transformed the war's material balance. By November 1918, over 2 million U.S. soldiers had arrived in France, with another 2 million in training. They arrived precisely as Allied armies were exhausted and German spring offensives had nearly broken the line. Fresh American troops plugged gaps that British and French forces could no longer fill.
The U.S. had been economically entangled with the Allies for years — providing loans, munitions, and food that German U-boats had been trying to cut off. American entry made that support official and unlimited. The weight of American industry and manpower made Allied victory nearly certain, though fighting would continue for another 19 months.