What role did the U.S. play in the Allied victory in World War I?

The United States did not win World War I for the Allied Forces - but they did play a major role in bringing the war to a close and sealing an Allied victory.

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The American Expeditionary Force went to the Western front in France. Led by General George Pershing, the AEF was fresh and enthusiastic, since they had not been fighting for three years in the trenches like the other Allies.  At first they served as replacement troops for Allied Casualties.  However, Pershing anted the AEF to fight as an independent unit. He felt that the Allied troops had become too defensive after fighting for three years, and he also wanted the U.S. to have a strong voice in negotiating once the war was over.  The AEF fought from April 1918 to the end of the war as a separate unit, under the general command of French marshal Foch, commander of all Allied troops in Europe.  
The AEF actually fought in three major campaigns before the armistice.  They arrived in Europe just in time, since Germany made a major offensive in the spring of 1918, and German armies were only 50 miles from Paris.  The AEF helped stop the German troops at Cantigny in France.  The AEF assisted in stopping German attacks at Chateau-Thierry and Beleau Wood in June, and the helped win the second battle of the Marne in July.  In September, they mounted major offensives against the Germans in the Saint-Milhiel and Meuse-Argonne Area.  While the AEF suffered extreme casualties in the two campaigns, the tide was definitely turning against Germany and the other Central Powers.

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The U.S. Navy also played a very important role in the Allied cause.  Once the U.S. entered the war, shipyard building grew exponentially due to help from US government.  The Navy transported troops, supplies, and food to Europe. The Navy employed a convoy system to travel overseas, using  merchant vessels surrounded by destroyers and cruisers.  This "Bridge to France" helped stop the problem of submarine warfare.  The Navy also helped in fighting U-boat warfare by using submarine chasers, and aircraft.  They laid down a 230 mile barrier of mines from Scotland to Norway to bottle up German U-boats as well, and German U-boat warfare was reduced greatly.

According to David Kennedy, Professor at Stanford University, "the actual military contribution of the United States to the fighting in the end of the conflict was absolutely minimal. In so far as the presence of America made a difference in Germany's decision to surrender, it was not because of success on the battlefield at the Meusse Argonne, or anywhere else for that matter. It was because the entrance of America into the war, and its demonstrated capacity to move its army across the Atlantic in huge numbers, now faced the Germans with the prospect of a virtual endless limitless supply of reinforcements that could be brought to the Allied side."

Danzer, Gerald A. et al. The Americans. Evanston, Illinois: McDougal Littell, 1998. 

PBS Online.  The Great War.  Public Broadcasting Service.  [online] Available at: http://www.pbs.org/greatwar/