<P> The United States declared war on Germany on April 6, 1917, over 2 ​ ⁄ years after World War I started . A ceasefire and Armistice was declared on November 11, 1918 . Before entering the war, the U.S. had remained neutral, though it had been an important supplier to Great Britain and the other Allied powers . </P> <P> The U.S. made its major contributions in terms of supplies, raw material and money, starting in 1917 . American soldiers under General John Pershing, Commander - in - Chief of the American Expeditionary Force (AEF), arrived in large numbers on the Western Front in the summer of 1918 . During the war the U.S. mobilized over 4 million military personnel and suffered 110,000 deaths, including around 45,000 who died due to the 1918 Spanish Flu outbreak (30,000 before they even reached France). The war saw a dramatic expansion of the United States government in an effort to harness the war effort and a significant increase in the size of the U.S. Armed Forces . </P>

Who did america fight with in world war 1