The entrance of the United States into World War I was a very significant event. The fighting in World War I had been going since the summer of 1914. By April of 1917, when the United States entered the war on the side of the Allies, neither side was making progress. Both sides in the war had suffered heavy casualties. Morale was low, as few people had believed the war would last this long, and it appeared there was no end in sight.

When the United States entered the war, it was a huge help to the Allies. The American soldiers were rested and brought energy to the Allies. The spirit of the Allied soldiers improved significantly. Unites States industries produced much-needed supplies for the Allies. Military equipment and food were provided to assist the Allies in their fight against the Central Powers.

World War I was essentially at a stalemate until the United States entered the war. The entrance of the United States into the war gave the Allies the edge they needed to defeat the Central Powers.

In short, American entry into World War I helped to bring the war on the Western Front in Europe to an end. The United States declared war in April of 1917, but was unable to commit significant forces (in the form of the American Expeditionary Force) until 1918. At this point, the American forces, fresh in comparison to all of the other forces on both sides in the field, helped to stop a massive German offensive in the spring. With the Germans severely weakened, the Americans played an equally crucial role in a final Allied offensive in the Meuse-Argonne region that drove the Germans back almost into Germany itself. This fall offensive set the stage for the end of the war. American involvement had a major effect on the diplomatic process that ended the war. Woodrow Wilson's Fourteen Points, issued early in 1918, were instrumental in encouraging a large faction in Germany that wanted to bring an end to the war. United States entry into the war was the biggest factor in bringing it to an end in November of 1918.

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.