The American Expeditionary Force (or "Forces") was the name applied to the American troops serving in Europe during World War I. When Congress declared War on Germany in 1917, the United States did not have the organization necessary for the deployment of the enormous numbers that would be required.
On May 26, 1917, General Pershing was instructed to take his staff to France. Shortly after arriving, Pershing cabled the War Department that he would require at least a million men by the following May and that there could be as many at three million eventually needed.
In response to the German attacks in the spring and summer of 1918, Pershing placed the entire American forces at the disposal of the Allied high command, delaying until July the formation of the American First Army. Two divisions of Americans deployed near Paris at Chateau-Thierry stopped the German advance. Approximately 300,000 American troops took part in the Second Battle of the Marne. More than 1,200,000 American soldiers ultimately fought in the war.
When the Armistice came, approximately two million American troops had been transported to Europe. By the end of August, 1919, the last American division had set sail, leaving only a small force in occupied Germany. Pershing and his staff embarked for America on September 1, 1919.