The United States entry into World War I was a turning point in providing money, munition, manpower and morale to help the Allies accomplish victory.
The United States had been providing loans and supplies to Britain and France prior to US entry into the war, but the commitment of the United States as a participant in the war brought the US financial backing and supply chain into full gear in support of the war effort. The knowledge that US backing and US troops were on their way also gave a huge morale boost to the European Allies as they pressed on to win the war. Finally, the arrival of American troops in Europe helped propel the Allies to final victory.