WWII

bigpigeon.us webpage WWII > WWII Ground Forces, updated by RAC 24 Oct 2022.
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The basic organizational structure of US WWII ground forces, from largest to smallest units, was as follows:
         Army Group > Army > Corps > Division > Regiment > Battalion > Company > Platoon > Squad.

Links to subpages:

• High-Level Organization - About Army Groups, Armies, Corps and Divisions.
• Infantry Division Organization - Marine Corps divisions as well as army Infantry divisions.
• Ground Forces - Japan - War with Japan High-level Ground Forces Organization.
• Ground Forces -  The MTO - The MTO High-level Ground Forces Organization.

​See also:

• The ETO > ETO Overview > ETO > Orders of Battle - ETO high-level ground forces organization on several dates.

Useful Order of Battle links:

• https://history.army.mil/documents/eto-ob/etoob-toc.htm - ETO divisional order of battle. Index includes organic composition of infantry, armored and airborne divisions. (I can't find analogous webpages for the MTO or the War with Japan.)
• ​https://www.armydivs.com - US Amy Divisions in World War II, includes sections for the War with Japan, the MTO, and the ETO; useful graphics and combat chronicles.

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Notes:

• Regimental-sized and larger combat units included smaller units from many Arms and Service branches, the Arms branches being Infantry, Artillery, Armor, and (sometimes at least) Engineers.
• Some organizational schemes had a section structure intermediate between a platoon and a squad.

Various ad-hoc combat organizations smaller than a division but generally larger that a battalion included:

• Infantry 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.
• Armored Combat Commands usually consisted of a tank battalion, an infantry battalion, and an artillery battalion. 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, an 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, often served with a division for extended periods.
• A WWII infantry division did not include armored vehicles; tank and antitank battalions were assigned as deemed necessary.

 
==> Using the above links, you may be able to do some web research - two examples:
Example 1 - Suppose you know your grandfather served in an infantry division in the ETO and you know his unit (e.g., 571st Signal Co.), but you don't know his division:

• Enter https://history.army.mil/documents/ETO-OB/IDs-ETO.htm
• You should see the Organic Composition of Infantry Divisions view of the ETO Order of Battle database.
• Search for 571.
• You should find that the 571st Signal Co. was in the 71st Infantry Division.

Example 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.

• Enter https://history.army.mil/documents/ETO-OB/ETOOB-TOC.htm
• You should see the table of contents for the ETO Order of Battle volume.
• Click on the hotspot for the 4th Infantry Division chapter.
• You should see the 4th Infantry Division chapter text.
• Scan through the text until you see the Attachments section, and within it the Armored subsection.
• Sure enough, the 70th Tank Bn. was generally attached to the 4th Infantry Division.

​Sources for Big Pigeon's WWII > Ground Forces webpage:

• ​There are no specific sources at this time. General sources are in the WWII >​ WWII Web Resources webpage.