The U.S. made major contributions to the war for a major effect despite coming in only one year before the Armistice was signed.
The weary allied soldiers enthusiastically welcomed American troops in the summer of 1918. They arrived at the rate of nearly 10,000 per day at a time when the Germans were not able to replace their losses. This shifted the momentum of the war from a near deadlock to a slow but steady offensive march toward Germany. The American army played a central role in the Hundred Days offensive that eventually forced the Germans to surrender at the end of 1918.
The U.S. Navy meanwhile helped guard convoys across the Atlantic. The also sent a battle group to the Scapa Flow to join the British Grand Fleet.
American largest contributions came at the peace table, where President Woodrow Wilson helped draft the Treaty of Versailles. Wilson also helped to create the League of Nations, forerunner to the modern UN. Although the U.S. Senate didn’t approve the final treaty, the U.S. still played a key role in global politics until the Great Depression focused them to look inside their own borders rather than beyond them.