bigpigeon.us webpage WWII > WWII Ground Forces > High Level Organization, updated by RAC 31 Jul 2022.
The high-level organizational structure of US WWII ground forces, from army group to division, was as follows:
         Army Group > Army > Corps > Division

About army groups in WWII:
About armies in WWII:
  • The US First, Third, Seventh and Ninth Armies fought in the ETO.
  • The Fifteenth Army was a late, rear-area army in the ETO that saw little combat.
  • The Fifth Army fought in Italy.
  • The Sixth, Eighth, and Tenth Armies fought in the War with Japan.

About corps in WWII:
  • Napoleon Bonaparte invented the corps, an organization intermediate between an army and a division.
  • Because of the large number of divisions, many WW II maps show actions by corps but not by division.
  • The US and British identified corps by roman numerals.
  • https://apps.dtic.mil/sti/pdfs/ADA445769.pdf contains information about all WWII US corps commanders.

About divisions in WWII:
  • The Marine Corps fielded six infantry divisions, with an amphibious corps as the highest organizational level, and fought only in the War with Japan.
  • The United states fielded so many divisions in WW II that to list them throughout this document isn’t feasible.
The six kinds of US army divisions :
  • Infantry - All infantry divisions had at their core three infantry regiments.
  • Light Armored - contained three armored infantry battalions, three tank battalions, and three armored artillery battalions. All light armored divisions served in the War with Germany.
  • Airborne - core consisted of two parachute infantry regiments and two glider infantry regiments.
  • Heavy Armored - only the 2nd and 3rd Armored Divisions, both of which served in the ETO.
  • Cavalry - only the 1st Cavalry Division, which served as infantry in the Southwest Pacific.
  • Mountain - only the 10th Mountain Division, which served in Italy.
I ignore the Heavy Armored, Cavalry, and Mountain divisions in the sequel.

Links to lists of army division-size and larger units:
Referring to this source, regarding WWII army infantry divisions:
  • The 1st - 9th, 12th, 24th, 25th, and 42nd  were Regular Army divisions.
  • The 23rd, 26th - 38th, 40th, 41st, and 43rd-45th were National Guard divisions.
  • The 63rd, 65th, 66th, 69th-71st, 75th-81st, 83rd-100th, 102nd-104th, and 106th were Reserve divisions. 
About specific infantry divisions:
  • The 12th Division, which included Filipinos serving in the US Army, was earlier known as the Philippines Division and was lost in the Fall of the Philippines in 1942.
  • The 23rd Division was earlier known as the Americal Division and was organized in New Caledonia in 1942 from surplus National Guard units.
  • The 34th Division consisted of National Guard units from Iowa, Minnesota, Nebraska and the Dakotas. The regimental headquarters and 1 1/2 battalions of the 168th Infantry Regiment were captured in Tunisia in 1943.
  • Two regiments of the 106th Division were captured early in the Battle of the Bulge.
  • The 92nd and 93rd Divisions were segregated; i.e. enlisted men were black and higher-ranking officers were white.