What allied country joined the war almost three years after the war began?

The United States joined World War I almost three years after the conflict began, officially entering the war on April 6, 1917. Initially neutral, the U.S. was drawn into the war due to factors such as unrestricted submarine warfare by Germany and the Zimmermann Telegram, which proposed a military alliance between Germany and Mexico against the U.S. The entry of American forces provided a significant boost to the Allied Powers and helped turn the tide of the war.