The US Military had some 16 million people in the Service during World War II, with over 70 Infantry Divisons counting Airborne... the 1st, 2nd, 3rd, 4th, 5th, 6th, 7th, 8th, 9th, 10th, 11th, 13th, 17th 24th 25th, 26th, 27th, 28th, 29th, 30th, 31st, 32nd, 33rd, 34th, 35th, 36th, 37th, 38th, 40th, 41st, 42nd, 43rd, 44th, 45th, 63rd, 65th, 66th, 69th, 70th, 71st, 75th, 76th, 77th, 78th, 79th, 80th, 81st, 82nd, 83rd, 84th, 85th, 86th, 87th, 88th, 89th, 90th, 91st, 92nd, 93rd, 94th, 95th, 96th, 97th, 98th, 99th, 100th, 101st, 102nd, 103rd, 104th, 106th and the 1st Cav.which was used as Infantry.
Not too shabby if you ask me.
By the time 1944 rolled around the US Military was just hitting its stride as far as training and combat operations, and had the War drug on the US Military would have grown even more to meet the need.