How did Americans affect the end of World War I and its peace settlements?

America broke the stalemate.

Until the entry of the U.S. in 1917, WW1 was in a stalemate. Each side literally traded yards of territory from one day to the next. There was no progress. The U.S. entry brought with it, of course, a large corps of soldiers. It was more a game of numbers than battles one once the U.S. arrived. Germany's position quickly became untenable and well before the armistice they were asking for peace.