US troops played a very important, pivotal role in ending World War I.
By the time the United States entered the war in 1917, both sides in this bloody, bitter conflict had fought themselves to a stalemate. But the infusion of fresh troops from across the Atlantic reinvigorated the Allied forces, providing them with much-needed men, material, and firepower that had the capacity to turn the tide in their favor.
The entry of the United States into the war also gave the Allies a massive boost in morale. As any general worth their salt will tell you, wars are not simply won by superior numbers or better technology; morale also plays a vital role in determining the outcome of any given conflict.
Once the Allied forces knew that the Americans would soon be coming to help them in their fight against the Central Powers they began to see, at long last, an end to a conflict that had raged for nearly three long years, leaving millions of dead in its inexorable wake. Inevitably, this lifted spirits and gave the Allies renewed hope that the enemy would soon be defeated.
Of course, sheer weight of numbers also played its part in bringing the war to a conclusion. Over a million American troops were sent to fight in Europe, and this vast amount of fresh manpower undoubtedly helped to tip the scales in the Allied forces' favor.
But the fighting quality of the American troops was also decisive. When the Germans launched their last big offensive in the war, there were still only less than a hundred thousand American troops in Europe.
Yet in the relatively short time they'd been there, they had already made a decisive contribution to the Allied war effort. As did the US Navy, which offered much-needed protection to Allied shipping from unrestrained German submarine warfare.
Without the involvement of American troops, it's highly unlikely that the First World War would have ended when it did. Instead, the war would have dragged on, costing more and more lives in a conflict that was, at that time, the bloodiest and most destructive the world had ever seen.