The U.S. troops arrived just in time to turn the tide of the war in favor of the Allies. Both sides were exhausted and running out of soldiers. The influx of fresh troops helped to boost the morale of the Allies and played a major role in the defeat of the Germans.
How did American entry into World War I affect the Allied Powers? It boosted their morale. During World War I, the zigzag pattern of trenches on the front lines was designed to protect troops from enemy fire. During World War I, most attacks in no-man's-land led to stalemate