In June 1966 the USMC were conducting Operation Jay, a sweep and clear operation hunting for VC forces in Quang Dien District. This is about 13 miles north of the famous city of Hue, that would reach the headlines during the Tet Offensive. The forces deployed included two battalions of marines and an artillery battalion. Towards the end of June the operation was being wound up with only a single battalion staying in the area, while the other forces were withdrawn.
However South Vietnam was also deploying a force of its marines (Vietnamese Marine Corp or VMC) to the area. This battalion was being moved up to the area on a convoy of twenty eight trucks. Route 1, which the VMC were taking, hadn't had any enemy activity on it for nearly a year. Despite this in anticipation of an attack the VMC were riding armed and ready for an ambush. The force had planned artillery fire along its route, and the command staff of the battalion brought along an observer. There was also a Vietnamese observer plane overhead. They left Hue city at 0730.
It took them an hour to reach the 0 Lau River, passing a column of USMC heading to Hue about five km from the city. Shortly afterwards the VMC convoy entered a series of rolling hills that was open to the west. To the east was a railway that ran parallel to the road, and was cut into the hillside. As the column reached the middle of the 3km stretch of open ground the enemy struck.
The enemy in this case was a VC battalion which had moved out the night previously. It seems to have had intelligence of the marines deployment as the VC moved out on the night of the 28th of June to be in position for the 29th, when the VMC column was due to pass. The USMC forces also had decent intelligence, as they received a warning of an impending attack on the night of the 28th. The USMC intelligence indicated the attack would be an assault against their units, not an ambush two and a half kilometres away against the VMC.
The VC battalion had set its heavy weapons up in the hills to the west of the road. As the VMC column entered the killing zone they opened fire, hitting the centre of the column with recoilless rifle and light mortar fire. Then machine guns and small arms opened fire raking the length of the column at a range of about 200m. Ten trucks were hit. Three were utterly smashed, two of which caught fire sending plumes of smoke into the sky. These smoke stacks were seen by the artillery battalion, who immediately leapt into action and swung their pieces through 180 degrees to be ready for any fire missions that came in. The sound of the gunfire was heard by the USMC convoy that had passed the VMC convoy. The USMC force was halted at a checkpoint, and upon hearing the racket they prepared for action, rolling up the canvas of their trucks, so they could fire out into the bush. They were thankful that they had driven through the VC killing zone and not been attacked.

Under the pounding the VMC deployed in an orderly fashion from their trucks and took up what little cover there was and began to return fire. Looking west there was a thin stand of trees which could be seen through, beyond this they could make out the black blasts and muzzle flashes of enemy weapons. Then they could see the VC starting to prepare an assault. The VMC commander immediately realised that the roadway had almost no cover and ordered his men to fall-back to the railway.
Two companies of the VMC started the fall back manoeuvre and crossed the 75m of ground without issues. Then when the lead elements reached the railway cut the VC sprung their second ambush. Hidden in concealed positions the VC Infantry had waited until the Vietnamese marines were at point blank range before opening fire. The first volleys killed the majority of the officers. The few marines that made it into the cut, and the dead ground from the line of VC infantry were taken under enfilading fire from the north by heavy machine guns sighted to fire along the line of the railway and cover the dead ground. The battalion commander (by now seriously wounded) had two companies on the road and two separated by 75m of fire swept open ground and he had lost control of the battalion.

USMC advisor's with the VMC battalion luckily had their own radios, and managed to contact a US Army observer plane nearby. The USMC artillery which had been listening in to the same net as the US army plane immediately offered fire support, which was gratefully accepted. The first salvo of shells impacted at 0846. Then the thunder began. Another observer plane arrived overhead, this one belonged to the USMC and on-board was a specialist Forward Air Controller. The FAC was in contact with F-4 Phantoms that were arriving in the area and he began to direct these strikes on the enemy.
At 0915 the increasingly crowded airspace was joined by the commanders of the USMC battalions that were conducting Operation Jay. They were carrying out reconnaissance of the ambush site prior to advancing. Both ARVN forces with tanks and the US Marines were closing on the ambush site. However the ARVN tanks couldn't reach the site due to a river being in the way. Not so the diminutive and much lighter M50 Ontos.
The Ontos was built to be bullet proof, with a great mobility. Armed with six M40 106mm recoilless rifles it had potent firepower. Originally 300 odd had been built, by the start of the US involvement in the Vietnam war the number in service was down to about half that number. With no logistical support they had to cannibalize older machines to make others work. Equally they had no spare tracks and so all the Ontos’s were dangerously worn. Despite this the Ontos’s had vastly superior mobility than most tanks. On this occasion the Ontos’s were able to cross a small rickety bridge and advance on the VC positions.
The Ontos had a reputation amongst the VC. It's bullet proof nature with horrific fire-power and unsurpassed mobility meant that it was rather feared. For example the 106mm recoilless rifle had a beehive round, each of these had 10000 metal flechettes inside it. So an Ontos could blanket an area with such fire-power that an reporter writing about the Battle Khe San said of the Ontos "[...] enough flechette ammunition to pin the entire North Vietnamese Army to the face of Co Roc Mountain".
Bundle of Beehive flechettes. Multiple of these would have been stacked inside the cone of the round.
In this particular case the Ontos platoon managed to get into position flanking the ambush. With airstrikes and artillery raining down, reinforcements arriving from both south and north, including a platoon of the feared Ontos, the VC broke and ran. The ones at the railway cutting had plenty of cover. However the force on the open ground to the west of the road lacked any such cover for their flight. One platoon waiting until it thought it was all clear left their position at speed hoping to get to more cover before another air strike or artillery could be directed at them. They'd forgotten about the Ontos’s. The Ontos platoon fired one round from each gun, and swept the VC platoon from existence.

The fight was all but over, with the forces west of the road being cut off by air mobile troops and surrounded. It cost the VC battalion 185 killed and most of its crew served weapons. The VMC had lost 42 dead.

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