American entry into World War I was quite a decisive force. It seemed that prior to the American entry, each side was doing a fairly good job of neutralizing the other. It was the American entry into the war that ended up proving to be decisive and would end up defining victory for the Allies. The production element made American entry so definitive, as European nations were struggling economically under the weight of war, American war production was very strong. Additionally, American military forces helped to decide the war in the Allies favor just out of sheer number. In a conflict where death was so prevalent, the injection of life from the American side helped to bring victory to the Allies.
Word War I started in June-July 1914, and USA directly entered the war nearly three years later by declaring war on Germany on on April 6, 1917. However even before U.S. help could reach the war fronts, the Allies had managed to overcome the U-boat threat in the Atlantic. In May 1917, Britain began to use a convoy system, by which cargo ships went to sea in large groups escorted by warships which was able to effectively neutralise effect of German U-boats. This improved their position substantially.
The troops sent by the US to Europe arrived in France in June 1917, and over a period about 2 million Americans served in Europe. Thus manpower was the chief contribution of the United States to World War I. This helped the allies to halt the string of successes of Germany on the western front and then defeat them on all the fronts, forcing them to accept an armistice on November 11, 1918.