I don't understand the difference between division, brigades and regiments etc in the US Army. Firstly, what's the difference and how does this compare to other major powers of the time? How many men were in each division etc. Sorry if this is a bit too much. Any effort to help me will be much appreciated :)
The brigade was an intermediate structure between the division and regiment, usually directly controlling two regiment-sized units. It was largely eliminated after 1940.
Infantry Divisions:
Prior to World War II, U.S. infantry divisions had a "square" structure, with four infantry regiments. These regiments were assigned to two brigades that reported to the division headquarters. There were also three artillery regiments (two light and one heavy, controlling a total of six battalions) that reported to the divisional artillery.
In 1940, a "triangular" structure was adopted that removed the infantry brigade as an organizational structure, as well as one of the infantry regiments; the three infantry regiments now reported directly to the division headquarters. The divisional artillery also saw major changes, with the artillery regiments being eliminated as an organizational structure; the 105 mm howitzer battalions were reduced from four to three, the 155 mm howitzer battalions from two to one, and they now reported directly to the divisional artillery instead of going through the artillery regiment. Other divisional units also saw similar changes. The strength of the initial triangular infantry division was around 15,200 men, in contrast to the 21,000 men of the post-World War I square division. The square division of World War I vintage was even larger, often having over 28,000 men! By the middle of WWII, a U.S. infsntry division could expect to field around 14,200 men.
The grouping of the major units of the infantry division into "threes" allowed the formation of regimental combat teams; an infantry regiment, a field artillery battalion, a company each of the division's engineer and medical units, and a company from a tank or tank destroyer battalion attached to the division.
"Orphaned" infantry regiments created through triangularization were often used to run training camps in the United States, were assigned to other units when necessary, or were deactivated (see below).
Unlike in the British Army, U.S. infantry regiments had a fixed number of organic battalions as dictated by their table of organization, and as a result, regiments as a whole could only be assigned to one division at a time. During World War I, infantry regiments numbered from 1 to 100 were marked for assignment to divisions of the Regular Army, regiments numbered from 101 to 300 were to be assigned to divisions of the National Guard, and those numbered 301 and above were to be assigned to divisions of the National Army (made up of draftees). After World War I, most of the divisions of the National Army were reconstituted as a partially-formed Organized Reserve. The draftee force was renamed the Army of the United States.
Regiments could be moved between divisions when necessary, and independent regiments existed. The 53rd Infantry Regiment was removed from the 7th Infantry Division to act as defenses on the West Coast in 1941, and was replaced with the 159th Infantry Regiment, a California National Guard unit from the 40th Infantry Division. The "gap" in the 40th Infantry Division was filled with the 108th Infantry Regiment, an excess regiment made homeless when the 27th Infantry Division (New York National Guard) was triangularized.
The 159th was ordered to stay in the Aleutian Islands after the 7th Infantry Division was victorious over the Japanese there in 1943, and it was replaced by the 184th Infantry Regiment, another excess regiment originally from the 40th Infantry Division. The 159th Infantry Regiment, along with the 103rd Infantry Regiment, were deployed to Europe in 1945 to reconstitute the 106th Infantry Division, which had been severely battered during the Battle of the Bulge.
The 147th and 158th Infantry Regiments and the 112th Cavalry Regiment fought as separate regiments in the Pacific. The 442nd Infantry Regiment, the most decorated regiment in United States military history, was a separate regiment made up of Japanese-American troops. During WWII, the United States Army experimented with using first-generation immigrants in units. As many, if not all, could speak their native language along with English, they could be useful as spies or when serving in occupied areas. Five separate battalions were created, but only two (the 99th, of Norwegians, and the 100th, of Japanese) saw combat.
| Infantry Division | Component | Regiments |
|---|---|---|
| 1st | Regular Army | 16th, 18th, 26th |
| 2nd | Regular Army | 9th, 23rd, 38th |
| 3rd | Regular Army | 7th, 15th, 30th |
| 4th | Regular Army | 8th, 12th, 22nd |
| 5th | Regular Army | 2nd, 10th, 11th |
| 6th | Regular Army | 1st, 20th, 63rd |
| 7th | Regular Army | 17th, 32nd, 53rd, 159th |
| 8th | Regular Army | 13th, 28th, 121st |
| 9th | Regular Army | 39th, 47th, 60th |
| 10th Mountain | Army of the United States | 85th, 86th, 87th |
| 23rd | Army of the United States | 132nd, 164th, 182nd |
| 24th | Regular Army | 19th, 21st, 299th |
| 25th | Army of the United States | 27th, 35th, 298th |
| 26th | National Guard (CT, MA, ME, NH, RI, VT) | 101st, 104th, 328th |
| 27th | National Guard (NY) | 105th, 106th, 165th |
| 28th | National Guard (PA) | 109th, 110th, 112th |
| 29th | National Guard (MD, VA, D.C.) | 115th, 116th, 175th |
| 30th | National Guard (GA, NC, SC, TN) | 117th, 119th, 120th |
| 31st | National Guard (AL, FL, LA, MS) | 124th, 155th, 167th |
| 32nd | National Guard (MI, WI) | 126th, 127th, 128th |
| 33rd | National Guard (IL) | 123rd, 130th, 136th |
| 34th | National Guard (IA, MN, ND, SD) | 133rd, 135th, 168th |
| 35th | National Guard (KS, MO, NE) | 134th, 137th, 320th |
| 36th | National Guard (TX) | 141st, 142nd, 143rd |
| 37th | National Guard (OH) | 129th, 145th, 148th |
| 38th | National Guard (IN, KY, WV) | 149th, 151st, 152nd |
| 40th | National Guard (CA, UT) | 108th, 159th, 160th, 185th |
| 41st | National Guard (ID, MT, OR, WA, WY) | 162nd, 163rd, 186th |
| 42nd | Army of the United States | 222nd, 232nd, 242nd |
| 43rd | National Guard (CT, ME, RI, VT) | 103rd, 169th, 172nd |
| 44th | National Guard (NJ, NY) | 71st, 114th, 324th |
| 45th | National Guard (AZ, CO, NM, OK) | 157th, 179th, 180th |
| 63rd | Army of the United States | 253rd, 254th, 255th |
| 65th | Army of the United States | 259th, 260th, 261st |
| 66th | Army of the United States | 262nd, 263rd, 264th |
| 69th | Army of the United States | 271st, 272nd, 273rd |
| 70th | Army of the United States | 274th, 275th, 276th |
| 71st | Army of the United States | 5th, 14th, 66th |
| 75th | Army of the United States | 289th, 290th, 291st |
| 76th | Organized Reserve (CT, RI) | 304th, 385th, 417th |
| 77th | Organized Reserve (NY) | 305th, 306th, 307th |
| 78th | Organized Reserve (NJ) | 309th, 310th, 311th |
| 79th | Organized Reserve (PA) | 313th, 314th, 315th |
| 80th | Organized Reserve (MD, VA, D.C.) | 317th, 318th, 319th |
| 81st | Organized Reserve (NC, TN) | 321st, 322nd, 323rd |
| 83rd | Organized Reserve (OH) | 329th, 330th, 331st |
| 84th | Organized Reserve (IN) | 333rd, 334th, 335th |
| 85th | Organized Reserve (MI) | 337th, 338th, 339th |
| 86th | Organized Reserve (IL) | 341st, 342nd, 343rd |
| 87th | Organized Reserve (AL, LA, MS) | 344th, 345th, 346th |
| 88th | Organized Reserve (IA, MN, ND) | 349th, 350th, 351st |
| 89th | Organized Reserve (KS, NE, SD) | 353rd, 354th, 355th |
| 90th | Organized Reserve (TX) | 357th, 358th, 359th |
| 91st | Organized Reserve (CA) | 361st, 362nd, 363rd |
| 92nd | Army of the United States | 366th, 370th, 371st |
| 93rd | Army of the United States | 25th, 368th, 369th |
| 94th | Organized Reserve (MA) | 301st, 302nd, 376th |
| 95th | Organized Reserve (OK) | 377th, 378th, 379th |
| 96th | Organized Reserve (OR, WA) | 381st, 382nd, 383rd |
| 97th | Organized Reserve (ME, NH, VT) | 303rd, 386th, 387th |
| 98th | Organized Reserve (NY) | 389th, 390th, 391st |
| 99th | Organized Reserve (PA) | 393rd, 394th, 395th |
| 100th | Organized Reserve (KY, WV) | 397th, 398th, 399th |
| 102nd | Organized Reserve (AR, MO) | 405th, 406th, 407th |
| 103rd | Organized Reserve (AZ, CO, NM) | 409th, 410th, 411th |
| 104th | Organized Reserve (ID, MT, UT, WY) | 413th, 414th, 415th |
| 106th | Army of the United States | 103rd, 159th, 422nd, 423rd, 424th |
Oddities:
The 23rd Infantry Division was made up "on the fly" of three National Guard regiments sent to defend New Caledonia in 1942, but was designated an Army of the United States division
The 24th and 25th Infantry Divisions were made from the disbanding Hawaiian Division; as a result, they were constituted with both Regular Army and Hawaii National Guard troops
The 42nd Infantry Division was a National Guard unit during World War I, formed from handpicked units from 26 states and the District of Columbia. None of its units re-formed during the interwar period, so it was reactivated as an Army of the United States division with troops from every state.
The 71st Infantry Division was an Army of the United States division, but had Regular Army infantry regiments assigned to it
The 92nd and 93rd Infantry Divisions were segregated units made up of African-American troops; the 25th Infantry Regiment was one of the original regiments of "Buffalo Soldiers" activated just after the American Civil War
The 71st Infantry Regiment was a National Guard regiment from New York, but it violated the 1-100 numbering rule for Regular Army infantry regiments
The 121st Infantry Regiment, National Guard troops from North Carolina, were assigned to the 8th Infantry Division, a Regular Army unit
The 320th Infantry Regiment was relieved from the 80th Infantry Division as excess, and later assigned to the 35th Infantry Division; the same occurred with the 81st Infantry and 82nd Airborne Divisions, which lost their 324th and 328th Infantry Regiments to the 44th and 26th Infantry Divisions, respectively.
