Which event precipitated the United States' entry into World War I?

The United States entered World War I primarily due to the Zimmerman Telegram and Germany's policy of unrestricted submarine warfare. The Zimmerman Telegram, sent in January 1917 from Germany to Mexico, proposed a military alliance between Germany and Mexico in the event that the U.S. entered the war against Germany. This telegram was intercepted and decrypted by British intelligence, leading to public outrage in the United States when it was revealed. Additionally, Germany's submarine warfare, especially the sinking of civilian and neutral ships, including the Lusitania, led to significant loss of American lives and heightened tensions between the U.S. and Germany. These factors combined created a situation where American public opinion shifted towards intervention, ultimately resulting in the U.S. declaring war on Germany in April 1917.