There were multiple plans for ground forces of varying sizes. These were made in anticipation of various situations that might occur after 1941. These situations revolved around the defeat of one or more of Germanys priciple enemies, the USSR and/or Britain. That is to say the few allies the US has the more ground forces it would need. In 1941 these several plans, or more accurately 'estimates' of requirement were refered to as "V Plans". They were as much a effort to connect industrial planning to possbile military requirements. The largest estimated the requirements for a ground force of 300 divisions, & a smaller plan for 200 were two of the early plans or estimates. When in mid 1941 serious planning for mobilization during 1942 through 1944 a ground force of approx 155 divisions was allowed for. As estimates altered in 1942 that goal was reduced to 120 & then to 100 at the end of the year. In mid 1943 the decision was made to halt creation of US Army divisions at 89.
The US population would have to be almost twice as large to be able to free up that many fighting men. If you have more soldiers, you also need more equipment, ammunition, ships, food, etc., so you need more civilian workers too. ..
Some of that is valid. Tho when you count up the total number of men & women involved in or supporting miltiary operations, civilian crews on cargo ships used in war zones, civlian technicians in war zones, and everyone else taken from normal civilian occupations for military service in or out of uniform in 1944-45 it was well over 5% of the population and over 10% of the working population.
The other factor here is the V Plans & subsequent plans did not allow for exclusive use of US inhabitants for military service, or industrial/agricultural production. The British had already proved by 1941 there were several million military aged males world wide who were willing to fight the Axis. Arming and transporting them was what the Brits had trouble with. The V Plans anticipated arming & otherwise using in military operations several million non US citizens, and on drawing in the industrial capacity of every location possible.
In fact a considerable number of French, Chinese, & others were armed and supplied by the US. A accurate count of the number of divisions equiped and sustained by the US by 1945 would include in part:
89 US Army
6 USMC
8 French from the 1943 agreement
3 Chinese used in Burma
2 Brazilian
1 Italian
That is 119 divisions equipped and sustained that were specifically identified in various agreements with other governments. There may have been a few others I've missed. On top of that there were a number of formations that were not specifically planned but drew largely from the US stocks of weapons and supply. In early 1945 this can be estimated as:
3 Soviet tank corps or division size armored formations with US tanks,
trucks, radios, ect...
3 Chinese divisions with US weapons, and other equipment
1 Italian forming
6 French taking delivery of US equipment of all classes
That is only a rough estimate from incomplete information. Another way to estimate this is from all US combat material and supplies shipped overseas but not used by US Ground Forces. A conservative estimate for late 1944 falls out at about 15 divisions worth of 'stuff'. More is possible depending on how the numbers are interpreted.
Bottom line here is the US was fielding over 130 divisions, which is a lot closer to the 150 goal laid out in 1942 than the 89 official US Army divisions.
It is also important to note how the US Air Forces and Army Service Forces in the early V Plans were not as large as those of 1944. The number of aircraft squadrons nearly doubled and Service Forces of units like port operations groups, railroad operations, air transport, army sea transport, ect... were far larger than originally estimated. Many of the missing divisions still existed in terms of manpower, but the formations had different titles and purposes.
A final point is US labor use had some large inefficencies through to the end. One of these was racist in nature. The Japanese descended population largely sat out the war in interment camps, contributing far less than they might have. African Americans were underused by the military and by industry. Many that could have moved to war related work were discouraged from that by riots, lynchings, and other violence and remained in marginal agricultural work. Latin Americans were recruited through some programs for military service and industrial work, but the programs left a large untapped pool both in the US and south of the border. These issues could have been forced to some degree had the incentive existed.
I could say the main role of the US infantry was telling the USAAF were the Germans where so the bombers could bomb them until it was safe for the GIs to move forward (or send some Polish troops first to see if it was really safe).
