W'e're going to jump back a few years to World War I. (Don't you love being a time traveler?)
The United States entered World War I on April 4, 1917 following a Declaration of War on Imperial Germany by Congress. In June 1917 the first American troops arrived in France – these were some 13,000 US Army regulars destined to form the 1st Infantry Division, and a battalion from the 5th Marine Regiment (during World War I, US Army troops and US Marines fought together in the US 2nd Infantry Division).
From this small beginning, the American Expeditionary Force (AEF) eventually grew to number over 2 million men in Europe by November 1918.