1. The sinking of the Lusitania. In May 1915, a German U-boat sank the British passenger ship Lusitania, killing over 1,198 people, including 128 Americans. This event caused a great deal of outrage in the United States and helped turn public opinion against Germany.
2. The Zimmermann Telegram. In January 1917, the British intercepted the Zimmermann Telegram, a secret message from the German foreign secretary to the German ambassador in Mexico. The telegram proposed an alliance between Germany and Mexico in which Germany would help Mexico regain its lost territory in the United States. The telegram outraged the American public.
3. The unrestricted submarine warfare. In early 1917, Germany began to wage unrestricted submarine warfare, meaning that it would attack any ship, neutral or otherwise, that was found in the waters around the British Isles. This policy led to the deaths of many American citizens and merchant ships.
In response to these events, President Woodrow Wilson asked Congress to declare war on Germany. Congress voted to declare war on April 6, 1917, and the United States officially entered World War 1.