The U.S. played a crucial role in World War I, initially remaining neutral until 1917. The entry into the war was prompted by factors such as unrestricted submarine warfare by Germany and the Zimmermann Telegram, which suggested a German-Mexican alliance. Once involved, American troops and resources bolstered the Allied forces, contributing to their eventual victory in 1918. The U.S. also participated in shaping the post-war peace settlement, notably the Treaty of Versailles. Ultimately, America's involvement marked a shift in its foreign policy, moving towards a more active global role in international affairs.