There were a number of factors that led to the United States' decision to enter World War I on April 6, 1917. These included:
* The sinking of the Lusitania. The RMS Lusitania was a British ocean liner that was sunk by a German U-boat on May 7, 1915. The sinking killed 1,198 people, including 128 Americans. The sinking of the Lusitania was a major turning point in the war, as it helped to turn American public opinion against Germany.
* The Zimmermann Telegram. The Zimmermann Telegram was a secret message from the German Foreign Secretary, Arthur Zimmermann, to the German Ambassador in Mexico, Heinrich von Eckardt. The telegram proposed an alliance between Germany and Mexico in which Mexico would attack the United States in return for German support in recovering the territories that Mexico had lost in the Mexican-American War. The telegram was intercepted by British intelligence and decoded. The release of the telegram to the public in March 1917 further inflamed American public opinion against Germany.
* Unrestricted submarine warfare. Germany's resumption of unrestricted submarine warfare in February 1917 was another major factor in the United States' entry into the war. Unrestricted submarine warfare meant that German U-boats would attack any ship, regardless of its nationality or whether it was carrying passengers. This led to the deaths of hundreds of American civilians and sailors, and it further intensified American anger towards Germany.
When did the US Join World War 1?
The United States officially declared war on Germany on April 6, 1917. The United States entered the war on the side of the Allies, which also included Great Britain, France, and Russia. The United States' entry into the war turned the tide in the war in favor of the Allies, and it helped to bring about the German surrender in 1918.