bigpigeon.us webpage World Wars > WW II Ground Forces, updated by RAC 12 Sep 2021.

As of 12 Sep 2021, this is a new webpage. The intent is to describe US Army and Marine Corps ground forces in WW II.

The basic organizational structure of US ground forces during WW II, from largest to smallest units, was as follows:
         Army Group > Army > Corps > Division > Regiment > Battalion > Company > Platoon > Squad.
Notes on the above:
  • Regimental-sized and larger units included smaller units from many Arms and Service branches, the Arms branches being Infantry, Artillery, Armor, and (sometimes at least) Engineers.
  • The Marine Corps fielded six infantry divisions, had a corps as the highest organizational level, and fought only in the War with Japan.
  • A non-infantry platoon was sometimes organized into sections rather than squads.

US Army Divisions - there were six kinds:
  • Infantry - All of the numerous 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 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.

Some will find the following source useful: Referring to this source, regarding WW II 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, consisting of American and Filipino troops and captured in 1942, was earlier known as the Philippines Division.
  • The 23rd Division was earlier known as the Americal Division.
  • The 34th Division consisted of National Guard units from Iowa, Minnesota, Nebraska and the Dakotas.
  • The 92nd and 93rd Divisions were segregated; i.e. enlisted men were black and higher-ranking officers were white.

Various ad-hoc temporary organizations smaller than a division but generally larger that a battalion included:
  • Regimental Combat Teams were built around an infantry regiment and supporting units. The best-known was the Japanese-American 442nd Regimental Combat Team, which fought in Italy and France.
  • Combat Commands usually consisted of a tank battalion, an infantry battalion, and an artillery battalion taken from an armored division. They were heavily used both in the Mediterranean and European Theaters.

Many company, battalion, or regimental combat units were independent in that they were not permanently assigned to a division, but were managed at the corps or army level.
  • An example was the 112th Cavalry Regiment of the Texas National Guard which served in the Southwest Pacific. My daughter's father-in-law, Lloyd Joyce, served in this unit. 
  • As the need arose, the army or corps commander would assign independent tank, tank destroyer, artillery, antiaircraft artillery, or engineer units to a division. These units, most often of battalion size, sometimes served with a division for extended periods.
 
A potentially useful document:
​Army infantry division organization.

Ignoring the headquarters company, each army infantry division contained the following organic (i.e., permanent) units:
  • three infantry regiments.
  • four field artillery battalions.
  • one each of a signal, quartermaster, and ordnance company.
  • a reconnaissance troop (troops were company-sized).
  • an engineer combat battalion.
  • a medical battalion.
​These organic units were each identified by a number; e.g. 168th Infantry Regiment is of special interest to me.

Using the above, you may be able to do some web research:
1 - Suppose you know your grandfather served in the ETO, you know his unit (e.g., 571st Signal Co.), but you don't know his division:2 - Suppose you know your grandfather served in the ETO, you know his unit (e.g., 70th Tank Bn.), and you've heard that he might have been in the 4th Infantry Division.
In WW II, the core of each Army infantry division (and each Marine Corps division) consisted of three infantry regiments, each regiment having three infantry battalions, and each battalion having four infantry companies:
  • First Battalion: Co. A (rifle), Co. B (rifle), Co. C (rifle), Co. D (heavy weapons)
  • Second Battalion: Co. E (rifle), Co. F (rifle), Co. G (rifle), Co. H (heavy weapons)
  • Third Battalion: Co. I (rifle), Co. K (rifle), Co. L (rifle), Co. M (heavy weapons)
Thus, if you know your ancestor's company number you can figure out his battalion.

Some will find the following links useful:
Army and Marine Corps divisional artillery organization.
​
Army Infantry:
  • Three battalions of 105-mm howitzers and one of 155-mm howitzers. Divisional artillery was towed rather than self-propelled.
  • In addition, each infantry regiment had a 105-mm howitzer cannon company and a 57-mm gun antitank company.
Army Light Armored:
  • Three battalions of self-propelled 105-mm howitzers mounted on the M7 Priest motor carriage.
Army Airborne:
  • Battalions generally equipped with 75-mm pack howitzers.
  • In WW II airborne operations, heavier equipment landed in gliders.
Marine Corps:
  • Four or five artillery battalions, organized into a 4th regiment (e.g., the 22nd Marine Regiment (artillery) was the 4th regiment in the 1st Marine Division).
  • Early in WW II, a marine division’s artillery regiment had three battalions of 75-mm pack howitzers and one of 105-mm howitzers.
  • As the war progressed, more 105-mm howitzers were added and the number of pack howitzers was reduced.

Links to additional Army and Army Air Force reports:
Sources for the WW II Divisions webpage:
  • The webpage header image depicts a full-strength WW II Army infantry division rifle squad and is courtesy of https://www.battleorder.org.