The United States had an impact on World War I before, during, and after the time that it was actually involved in the war.
Before the US was involved in the war, both sides wanted to keep the US happy. They did not want the US entering the war on the side of their opponents. This led, most notably, to the temporary suspension of unlimited submarine warfare on the part of Germany. Germany curtailed their submarine warfare for fear of provoking the US.
Once the US entered the war, it had a decisive effect on the fighting. This is because the two sides were at a complete stalemate before the US got involved. The influx of a huge number of fighting men and their materiel swung the war in favor of the Allied Powers.
After the war ended, President Wilson’s “14 Points” proposals had an impact on the peace process. Wilson pushed for an idealistic peace treaty, including the creation of the League of Nations. He did not get everything he wanted by any means, but his proposals did help to shape the debate and they did affect the Treaty of Versailles to some degree.