The U.S. officially entered World War I on April 6, 1917. Prior to this, the U.S. had maintained a position of neutrality, although it was economically and politically involved in the conflict. Factors such as unrestricted submarine warfare by Germany, the sinking of ships like the Lusitania, and the interception of the Zimmermann Telegram, which proposed a German-Mexican alliance, swayed public opinion and government policy. The entry of the U.S. provided substantial military and economic support to the Allies, significantly impacting the war's outcome and reshaping international relations in the post-war era.