The United States entered World War I in 1917, helping the exhausted Allies and changing how Americans viewed their country and the world. The American army quickly grew from 200,000 to over one million soldiers. Despite initial skepticism from French and British commanders, American troops showed valor in key battles like Belleau Wood and St. Mihiel, influencing the war's outcome. The war brought rapid changes at home and on the battlefield, marking a significant moment in American history.