Why did american entry into wwi impact the war

Sinking of American merchant ships. In early 1917 Berlin forced the issue. Its declared decision on 31 January 1917 to target neutral shipping in a designated war-zone became the immediate cause of the entry of the United States into the war. Five American merchant ships went down in March.

Since the U.S. troops entered the war. It really boosted the morale of the Allied European forces. Many say that if America did not join in, the allies would probably have lost. Also, Patton and those really awesome leaders kicked Central Powers in the butt and showed them America is boss.

This is all on the top of my head, but what's really important is the morale thing

The fact that farmers were paid for destroying crops.