To Expand on Abers reply... Information is from Stauntons 'OB of the US Army in WWII'. Shown below also is info on when the divisions were "split" to provide cadres for new divisions. It was possible to provide those cadres without splitting, but significant delays would havve been incurred in training those cadres & the new formations. There were also issues in retraining the divisions when they were converted from square to triangular in 1940-41. For some the conversion/combat readiness was not complete until 1942.
............Activated...........
1 Cav..... 1921
2dCav.....1941.................Contributed cadres for 8AD 4/1942,, 9AD 4/1942, 93ID 5/1942 2d Cav inactivated 8/1942. Reactivated 3/1943
Pillipine..1921
1st.........1917.................76th ID 6/1942
2d..........1917.................85th ID 5/1942.......2d split again for...102d ID 9/1942
3rd.........1917.................81st ID 6/1942
4th.........1940.................79th ID 6/1942.......4th split again for...84th ID 10/1942
5th.........1939.......Sent to Iceland early 1942
6th.........1939.................90th ID 3/1942.......6th split again for...89th ID 7/1942
7th.........1940.................95th ID 7/1942.......7th split again for...99th ID 11/1942
8th.........1940.................85th ID 5/1942.......8th split again for...102d ID 9/1942
9th.........1940.................82d ID 3/1942.......8th split again for...88th ID 9/1942 Note 82ID redesignated Airborne 8/42
24th.......10/1941... Original Hawaiian Division
25th.......10/1941... Formed out of miscl NG units in Hawaii & other from US.
Americal..5/1942 out of miscl separated units sent to S pacific in early 1942
Of the 17 new divisions shown above 14 were split again in 1942-43 for new divisions. Part of the splitting included the fourth regiment and two brigade HQ removed from the original divisions removed for conversion from 'square'.
26- 45 excepting 39 were National Guard divisions activated from October 1940 through March 1940. 18 total. NG divisions were not split like the others, but did have a portion of their individual cadres bled off to other other units, plus the fourth regiment and two brigade HQ removed for conversion from 'square'
1AD.........1940.................6 AD 2/1942
2AD.........1940.................6 AD 2/1942
3AD.........1941
4AD.........1941.................8 AD 1/1942....Provided cadres for 10AD 7/42, 11AD 8/42, 12AD 9/42, 13AD 10/42, 14AD 11/42, 20AD 3/43
5AD.........1941.................6 AD 1/1942
7AD.........1/1942
16AD.......7/43
The dates shown above are official activation dates. Key personel, the core cadre, were identified by name 4-6 months before the activation date & were sent to appropriate schools for their new job. This was usually one level above their previous job. ie: A major as assistant G3 in the 6th ID would become the primary G3 officer in the 90th ID. Cadres were also bled off from the divisions to form Corps and Army HQ staff.
Anyway, US Army formations sent overseas before November 1942 are liable to have ripple effects on activating and making ready other formations in 1943.
Note: There were two reorganizations of the infantry divisions after the 1940 mobilization. First was conversion from square divisions to a triangular organization based on prewar experimentation. Second occured in 1942 & was based on lessons learned and new equipment available. The second 'lightend' the ID by reducing the over all personnel numbers over 10% while keeping the rifle squad strength intact. This second reorg. in 1942 also delayed readiness of many units.