Why did the U.S. enter World War I?

The United States entered World War I primarily because of German submarine warfare against American shipping and a broader commitment to defend democratic governments against autocratic powers. From the start of the war in 1914, the United States traded heavily with both sides, but a British naval blockade meant that most American goods flowed to the Allies. Germany responded with submarines, called U-boats, and began sinking Allied and neutral merchant ships.

On May 7, 1915, a German U-boat torpedoed the British liner Lusitania off the coast of Ireland, killing 1,198 people, including 128 Americans. President Woodrow Wilson protested fiercely, and Germany temporarily restricted submarine attacks. That changed on January 31, 1917, when Germany announced unrestricted submarine warfare, meaning U-boats would sink any vessel in a declared war zone around Britain, including neutral ships, without warning. German U-boats sank several American merchant ships in the following weeks.

A second trigger came in February 1917, when British intelligence intercepted and decoded the Zimmermann Telegram, a secret message from German Foreign Secretary Arthur Zimmermann offering Mexico an alliance and the return of Texas, New Mexico, and Arizona if Germany won. The publication of the telegram on March 1, 1917 outraged Americans and made German hostility impossible to ignore.

A third reason was ideological. Wilson believed the United States had a duty to defend representative government, and after Russian Tsar Nicholas II abdicated in March 1917, the war could be presented as a contest between democracies and autocracies. In his April 2, 1917 war message, Wilson told Congress that the world must be made safe for democracy.

Economic ties also mattered. American banks had loaned the Allies more than two billion dollars by 1917, and an Allied defeat would have been disastrous for the United States economy. Other factors included sympathy for British and French culture, propaganda highlighting German atrocities in Belgium, and the desire to influence the postwar peace settlement. Together these forces overcame the strong American tradition of neutrality and convinced Congress to declare war on April 6, 1917, sending more than two million American troops to fight in France over the next year and a half.