The United States entered World War I primarily as a result of the sinking of the British ocean liner RMS Lusitania in May 1915, which killed 128 American passengers. However, it was the interception of the Zimmermann Telegram in January 1917 that galvanized public opinion against Germany. In this telegram, Germany proposed a military alliance with Mexico if the U.S. entered the war against Germany, promising to support Mexico in reclaiming lost territories. This, along with unrestricted submarine warfare by Germany, contributed to Congress declaring war on April 6, 1917. American involvement was pivotal in turning the tide in favor of the Allies.