1. Unrestricted submarine warfare: Germany's unrestricted submarine warfare campaign, which resulted in the sinking of American ships and the loss of American lives, was a major factor in the US decision to enter the war.
2. Zimmermann Telegram: The Zimmermann Telegram was a secret message from Germany to Mexico, intercepted and decoded by British intelligence, that proposed an alliance between the two countries and promised Mexico the return of territories lost to the US in the Mexican-American War if Mexico would attack the US. This revelation angered many Americans and helped turn public opinion in favor of war.