Under the right conditions the NVA/VC would spring a full scale, five-element ambush. The three primary elements of such an ambush were the lead blocking element, the main assault force and the rear-blocking element.
Lead Blocking Element
The lead blocking element, usually 10 percent of the total enemy force, had but one mission. They were to stop the enemy unit by inflicting as many casualties and by causing as much confusion as they could. It was essential that the enemy lead element was halted and in order to achieve this they would be firmly and strongly engaged. Carefully designed fire lanes were usually sighted so that both enfilading and converging fire could be poured into the kill zone. Maximum firepower would be generated by the lead-blocking element against the front of the enemy, stunning, and stalling him. In a battalion-sized ambush, the lead-blocking element would consist of a platoon reinforced by mines, one or two recoilless rifles, one or two HMG's and 60mm or 82mm mortars.
Rear Blocking Element
The rear blocking element, ten to twenty percent of the total ambush force, was targeted against the rear of the enemy column. Often 60mm and 82mm mortars would put pre-registered fire on the ambushed unit. If the entire enemy force was trapped within the kill zone, the rear-blocking element attacked the enemy rear. If the rear of the enemy force were not caught in the ambush, the rear-block unit would move to cut off the enemy rear from the rest of the column. The rear block protected the flank and rear of the close assault force and coordinated it's actions with that main maneuver element. If possible, the rear block would simultaneously neutralise the enemy rear while cooperating with the close assault force in an envelopment of that part of the enemy column trapped in the kill zone.
Main Assault Force
Sixty to eighty percent of the ambushing unit was deployed as the close assault element. That element massed to the flank of the enemy formation's line of march and was committed after the lead and rear blocks went into action. Reinforced with mines, antitank weapons, machine guns and, occasionally, flamethrowers, the close assault element struck the enemy column when it was most disorganised. In some cases, part of the assault element was held back as a reserve and later employed as a rear guard during the withdrawal phase.
Attacking from several directions near the center of the enemy column, the close assault element engaged the with the intention of initiating hand-to-hand combat and was charged with annihilating the enemy force.
The attackers intention was to fragment the enemy column and defeat it in detail within the killing zone. Each enemy fragment was then surrounded and wiped out.
The ambush commander, directing the battle from the OP near the staging area for the close assault element, sometimes chose not to commit it. Instead he might use it to reinforce the fires of the other two elements, or to carry out some other mission.
If at any time, the situation deteriorated to the point that a sizeable number of the ambushing unit was in danger of being itself annihilated, the commander signaled a withdrawal of some, or all, of his forces using a combination of flares, whistles, bugles, and/or voice commands. The withdrawal itself was sometimes used as a vehicle for laying additional deliberate or hasty ambushes of delay or annihilation.
The Mop Up
If the close assault unit successfully executed its mission by destroying the enemy main force, the battlefield was rapidly mopped up. All friendly dead, wounded and weapons were removed whilst the enemy dead were looted of their weapons, ammunition and documents. Prisoners were either hastily taken away along the withdrawal route or summarily executed.
A fast withdrawal over pre-selected routes then began with a rearguard consisting of approximately 10-20 percent of the ambush force seeking top cover the withdrawal by delaying actions such as sniping and 'Bloody Nose' ambushes.
Friendly dead were buried in pre-dug graves along the withdrawal route if time was available. Some corpses were often booby-trapped in order to catch unwary US troops engaged in body counts.
If necessary, the ambushing unit would divide into numerous smaller units, which then left the area in various directions to rendezvous later at pre-designated locations according to a predetermined time schedule.
Secrets of the Viet Cong J W McCoy, Hippocrene Books 1992, ISBN 0-7818-0028-5
Inside the VC and the NVA Michael Lee Lanning & Dan Cragg, Ivy Books 1994, ISBN 0-8041-0500-6