The United States participated in World War I from 1917 to 1918. Initially neutral, the U.S. entered the war in April 1917 due to a variety of factors, including unrestricted submarine warfare by Germany and the revelation of the Zimmermann Telegram, which proposed a military alliance between Germany and Mexico. American involvement shifted the balance in favor of the Allies, providing crucial military support and resources. The U.S. played a significant role in key battles, contributing to the eventual defeat of the Central Powers. The war ended with the signing of the Armistice on November 11, 1918, profoundly impacting U.S. foreign policy in the years to come.