The armored units which were deployed to RVN had been organised in line with US Army doctrine which anticipated highly mobile engagements with Soviet forces on the wide open plains of Central Europe. They were not organized for fighting a counter-insurgency war in SE Asia. As a consequence of this, extensive modifications were made to the standard Tables of Organization and Equipment.
One of the primary changes in doctrine involved the relationship between the roles of armor and infantry. In Europe it was envisioned that mechanized infantry units would accompany rapid armor columns in order to exploit gains made by the armor or to be ready to dismount and fight alongside the armor where necessary. This doctrine placed great emphasis on the primacy of the tank. The armored personnel carrier was viewed quite simply as an armored 'taxi' for the deployment of the infantry forces, enabling them to accompany the armor as it rapidly advanced.
In RVN this doctrine was pretty much reversed. In a combat environment which, for the most part, involved operations which were not of the European pattern due to their being no armored enemy, and in terrain wholly alien to the plains of Europe, the infantryman became the primary combat element and armor was relegated to either a static or supporting role.
In order to accommodate this shift in emphasis, armored units were modified to include a much greater proportion of infantry fighting vehicles, of which, the now ubiquitous M-113 and it's variants came to the fore. Infantry support was to be provided by the tanks and support-weapon armed APC's of the modified battalions and squadrons.
US Armored Battalion Modified TO&E (MTOE 17-35E)
See Also: Tank Company
Armored Cavalry Squadron Modified TO&E (MTOE 17-106/107 G and MTOE 17-78 G)
The standard TO&E consisted of 45 officers, 36 warrant officers (primarily helicopter crews), 824 enlisted men, 27 M-41 tanks, 75 M-113's, 6 M-577's, 5 recovery vehicles, 9 OH-6A, 11 UH-1B and six UH-10
The MTOE for Vietnam is substantially different from the Standard TOE – notice in particular the addition or upgrading of offensive vehicles (M-125, M-132, AH-1G and M-48’s). Also of note is that a substantial number of the M-113 were modified to ACAV’s.
For further information see US Divisional Armored Cavalry
TROOP D = Air Cavalry Troop and the disproportionate number of WO’s reflects the aviators within the troop.
Vietnam Order of Battle, Shelby Stanton, US News Books