What event caused the US to enter World War I?

The United States entered World War 1 because a German U-boat sank the Lusitania off the coast of Great Britain. There were some 128 American citizens killed. Many American ships had been sunk before this, but President Woodrow Wilson really didn't want to go to war with Europe. The Lusitania was sunk in 1915, a year into World War 1, and Wilson was reelected on a peace platform in 1916.

Public opinion had already turned against Germany in 1917 when the British released an intercepted diplomatic message, the Zimmerman Telegram, that showed Germany trying to incite Mexico into war with the US.

By this time, the American people finally had enough with the Germans sinking American cargo ships on their way to Britain with supplies for the war. Congress declared war on the Central Powers on April 6, 1917.