Unrestricted Submarine Warfare: Starting in February 1915, Germany implemented a policy of unrestricted submarine warfare, sinking civilian ships without warning in an effort to cut off vital supplies reaching Britain and its Allies. This tactic resulted in the loss of American lives and heightened public outrage in the U.S.
Zimmermann Telegram: In January 1917, the British intercepted and decoded a telegram sent by German Foreign Secretary, Arthur Zimmermann, proposing an alliance between Germany and Mexico. The telegram promised Mexican military support in exchange for Mexico regaining its territories lost to the U.S. in the Mexican-American War. The revelation of Germany's attempt to draw Mexico into the war on its side further inflamed American public opinion.
Sinking of the Lusitania: In May 1915, a German U-boat sank the British passenger ship Lusitania off the coast of Ireland. The ship carried over 1,200 passengers, including 128 American citizens. The loss of American lives in the sinking fueled public demands for the U.S. to take decisive action against Germany.