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 FLEXIBLE RESPONSE company. The fourth rifle company provided a unit for base defense and allowed the battalion to operate with three companies outside the base camp.

Combat brought about several changes in the infantryman's weapons. The light M16 rifle became the standard individual weapon, and a one-man light antitank weapon (LAW) often replaced the heavy and awkward 90-mm. recoilless rifle. Because of the nature of the fighting, heavier infantry weapons, such as the ENTAC, 4.2-inch mortar, and 106-mm. recoilless rifle, saw little service. When used, both the 81-mm. and 4.2-inch mortars were usually "slaved" to fire direction centers at American fire bases. Units did not suffer a loss of effective firepower because their mobility allowed them to concentrate their remaining weapons, while improved field radio communications aided in putting tremendous amounts of supporting fire at their disposal. Given organic, attached, and supporting aviation and signal units, all divisions and brigades had extensive airmobile and communications capabilities.

Although only one armor company, equipped with 90-mm, self-propelled antitank guns, assigned to a brigade and three divisional armor battalions, equipped with M48A3 tanks, served in Vietnam, divisions and brigades there had considerable armor. Each divisional reconnaissance squadron, except for the two in the airmobile divisions, had tanks and reconnaissance vehicles. The latter carried additional machine guns and gun shields, permitting the reconnaissance squadrons to function as armor. Also, the eight mechanized infantry battalions in Vietnam frequently performed as light armor units, using modified armored personnel carriers, By 1969 some reconnaissance and mechanized infantry units employed Sheridans, the M551 armored reconnaissance assault vehicles, in place of the light tank and armored personnel carriers. The Sheridan filled the need for a light tracked vehicle with greater firepower than the M113 armored personnel carrier.

Artillery, the third combat arm assigned to divisions and brigades, also underwent modifications in Vietnam. In the two airmobile divisions, a 155-mm. howitzer battalion was permanently attached after the 1st Cavalry Division demonstrated that the heavy howitzer could be moved by helicopter. Because of the large operational areas of divisions and separate brigades, their direct support artillery battalions often had four firing batteries, which were created in various ways. In the 173d Airborne Brigade, a fourth battery was authorized; in the 23d Infantry Division, each direct support battalion consisted of two five-gun and two four-gun batteries; and in the 1st Infantry Division, one or two 4.2-inch mortar platoons were attached to direct support artillery battalions as Batteries D and E.

By mid-1969 the seven divisions and four separate brigades in Vietnam reached their final configuration (Table 28). The ROAD building-block concept worked well, particularly in a war that was fought by brigades with divisions serving in a corps-like role. The Army, however, had difficulty meeting the demands of commanders for more tactical maneuver units because there was no pool of separate battalions to draw upon when needed for additional support.