- The sinking of the British ocean liner Lusitania by a German U-boat in May 1915, which killed 128 American citizens among the 1,198 fatalities. This event caused a major public outcry in the US and increased support for intervention.
- The Zimmermann Telegram, a secret message from the German Foreign Ministry to the German ambassador in Mexico, was intercepted and decoded by British intelligence in January 1917. The telegram proposed an alliance between Germany and Mexico against the US, with Mexico promised the return of territories lost in the Mexican-American War. This revelation further inflamed American public opinion and increased calls for intervention.
- The unrestricted submarine warfare policy adopted by Germany in early 1917, which sank American merchant ships without warning, led to a direct conflict between Germany and the United States.
These actions, combined with the US's growing economic ties to Britain and France and the desire to uphold democratic values against autocratic Germany, led the US to declare war on Germany on April 6, 1917.