Redeploying Marine divisions from the Pacific Theater would have hamstrung or even stopped operations conducted by the US Navy in the Pacific Ocean Area for a significant period of time. The Assault Training Center which was to be used for the training of the units which would take part in the D-Day landings was established in England in September 1943. Preparation for exercises began in November 1943. The Marine units that would have participated would need to have been redeployed through the United States and retrained in England. When exercise planning began at the Assault Training Center, the 1st Marine Division was preparing to invade Cape Gloucester, (they would stay until January 1944) the 2nd Marine Division was licking its wounds from the Battle of Tarawa, (they would next see action on Saipan at the same time as the D-Day landings) and the 3rd Marine Division was preparing to invade Bougainville (they would stay until January 1944) The 4th (August 1943) and 5th (November 1943) Marine Divisions had just been formed, and the 6th was not activated yet (September 1944)
| Exercise | Dates | Major Units |
|---|---|---|
| SAVVY | Unknown | 8th Canadian Infantry Brigade |
| PIRATE | October 1943 | 7th Canadian Infantry Brigade |
| EXCELSIOR I-VI | 5 December 1943-20 February 1944 | Artillery planning exercises |
| NUDGER | 7 December 1943 | LST loading exercise |
| DUCK I | 4 January 1944 | 175th RCT, 1st Engineer Special Brigade |
| DUCK II | 14 February 1944 | 116th RCT, 1st Bn, 531st Engineer Shore Regiment |
| CRACKSHOT | 19-25 February 1944 | Medical exercise |
| CHEVROLET | 25 February 1944 | Marshaling exercise (5th Engineer Special Brigade) |
| SNIPE | 27-29 February 1944 | LST loading and engineering exercise (1st Engineer Special Brigade) |
| DUCK III | 29 February 1944 | 115th RCT, 1st Engineer Special Brigade, tank and tank destroyer units |
| JEEP | 5-31 March 1944 | Marshaling exercise (38th Infantry Regiment, 2nd Infantry Division) |
| FOX | 9-10 March 1944 | 16th RCT, 116th RCT, 37th Engineer Combat Battalion, 149th Engineer Beach Group, 743rd Tank Battalion, 9th Air Force |
| MUSKRAT I (BEAVER I) | 13-23 March 1944 | 12th RCT |
| GULL | 15-18 March 1944 | LST loading exercise (1st Engineer Special Brigade) |
| OTTER I (BEAVER III) | 15-18 March 1944 | 3rd Bn., 8th RCT |
| MINK I (BEAVER V) | 15-18 March 1944 | 1st Bn., 22nd RCT |
| MINK II (BEAVER VI) | 19-22 March 1944 | 2nd Bn., 22nd RCT |
| OTTER II (BEAVER IV) | 19-22 March 1944 | 1st Bn., 8th RCT |
| MUSKRAT II (BEAVER II) | 24-26 March 1944 | 12th RCT, 1st Engineer Special Brigade |
| BEAVER VII | 27-30 March 1944 | 8th and 22nd RCTs, 1st Engineer Special Brigade, 502nd PIR, 9th Air Force |
| JALOPY | April 1944 | Marshaling exercise (10th (5th Infantry Division) and 121st (8th Infantry Division) Infantry Regiments) |
| SPANDU | 4 April 1944 | Signal Corps exercise |
| SPLINT | 12 April 1944 | Medical exercise |
| BOOMERANG | 19-30 April 1944 | 9th Air Force exercise |
| TIGER | 22 April 1944 | Full scale invasion rehearsal |
| FABIUS I-VI | 23 April 1944 | Full scale invasion rehearsal |
| CURVEBALL I/II | 27 April-6 May 1944 | 82nd and 101st Airborne Divisions |
| CELLOPHANE | 28 April 1944 | Loading exercise |
| PIGEON | 7-10 May 1944 | Communication rehearsal for Utah Beach |
| HAWK I/II | 14-18 May 1944 | VIII Corps artillery exercises |
The fundamental structure and logistics of employing Marine divisions was also in question. Marine divisions, unlike Army divisions, were not suited for long-term or rapid overland combat. They lacked any kind of long-range reconnaissance vehicles or units, and had relatively pitiful antitank assets when compared to German armor. The E-Series (TO&E E-100) Marine division, what would have been the most likely division type landed on D-Day, had 12 M3 Gun Motor Carriages, 54 37 mm antitank guns, and 243 bazookas. In comparison, a US Army division of early 1944 had 54 57 mm antitank guns and well over 500 bazookas. The E-series Marine division was authorized 54 light tanks in an organic tank battalion. Marine divisions were not assigned organic (the 1st Corps Tank Battalion (Medium) at Tarawa was a separate unit, designed to be broken up and assigned to Marine divisions' light tank battalions) medium tanks until the F-100 reorganization of May 1944. Marine tank operations conducted in the Pacific were fundamentally different than those that were to be undertaken in Europe, and retraining and re-equipping in the United States would be necessary.
Considering all of this, re-equipping Marine divisions for prolonged land fighting (and more along the lines of an Army division) would have been a moot point; they would clog up the carefully-planned logistics chain of the invasion as they would most likely need to be withdrawn after several days once the beachhead was secure. Given the timetable, it seems unlikely that entire Marine divisions would even need to go ashore. Employing separate Marine regiments detached from divisions instead of entire divisions for the initial assault would have been a simpler idea if any hypothetical consideration was made to using them at all.
Sources:
The American GI in Europe in World War II: D-Day: Storming Ashore, Volume 3, by J.E. Kaufmann and H.W. Kaufmann