The sinking of the British ocean liner RMS Lusitania by a German U-boat in 1915, resulting in the deaths of 128 Americans, created tension between the U.S. and Germany. The final straw was the interception and decoding of the Zimmerman Telegram in 1917, in which Germany proposed a military alliance with Mexico against the U.S., prompting the United States to join the war on the side of the Allies.
