Operation Francis Marion began on 6 April 1967 as a continuation of Operation Sam Houston across the same ground — Pleiku, Darlac and Kon Tum provinces. Peers, who had taken the 4th Infantry Division in January, designed a defense in depth against People's Army of Vietnam units moving out of Base Areas 701 and 702 inside Cambodia. His 1st Brigade deployed in an arc twenty kilometers east of the border, anchored on Highway 14B and on a north-south line of Special Forces camps at Plei Djereng, Đức Cơ and Plei Me, with the 2nd Brigade in reserve. Forward of that arc, Special Forces teams, long-range reconnaissance patrols and Civilian Irregular Defense Group units — mostly Jarai and other highland men recruited, armed and paid by the Americans — searched the ground out to the border. South Vietnamese divisions extended the screen north into Kon Tum and south into Darlac. On a map it looked like a fence. The border country is not a map.

On the morning of 18 May, Company B of the 1st Battalion, 8th Infantry was patrolling the densely jungled Ia Tchar Valley near the border when it saw a lone PAVN soldier and gave chase — into an ambush by a much larger force. The company formed a perimeter and called for everything it could, but the canopy was so thick that aircraft could not see the fight beneath it and absorbed much of the incoming artillery. Company B's 4th Platoon radioed that it was being overrun and brought artillery down on its own position. Their platoon sergeant, Sergeant First Class Bruce Alan Grandstaff of Spokane, Washington, was 32. He ran into open fire to reach wounded men, marked targets with smoke for the gunships, destroyed a machine gun position with grenades, and was mortally wounded by a rocket after directing the artillery onto himself. When Company A reached the platoon the next morning, 19 men were dead, one missing, seven wounded. The survivors said they had lain still among the bodies while PAVN soldiers shot those who moved.

Two days later, on 20 May, two more soldiers of the same battalion reached the same answer. Private First Class Leslie Allen Bellrichard of Janesville, Wisconsin, 25, was throwing grenades from a foxhole when a mortar round knocked an armed grenade out of his hand next to four of his comrades. He fell on it, and went on firing his rifle until he died. Staff Sergeant Ferenc Zoltán Molnár of Logan, West Virginia, 24, had already carried ammunition through open fire to his squad when a grenade landed among them; he covered it with his body. Three posthumous Medals of Honor, one battalion, three days — and all three for the same act, accepting death to buy the men beside them a few more seconds. It is a statistic that says less about heroism than about what that jungle was like to fight in.

The pattern held all summer, and the 4th Division's own analysts said it plainly: the PAVN chose the place and the timing. They struck columns moving through jungle or units settling into night positions, opened with heavy mortar and RPG fire, and broke contact when American firepower began to tell. Almost every engagement happened close enough to Cambodia that they could withdraw into sanctuary. Peers eventually had three brigades on the border, including the 173rd Airborne, and the PAVN still dropped mortars on Đức Cơ camp on 19 and 23 June. Weather did the rest. On 12 July two surrounded platoons had nothing but artillery keeping them from being overrun until the fog lifted at eleven; 31 Americans died there and seven went missing. Six of those became prisoners and did not come home until 1973.

The operation ended on 11 October 1967. US forces claimed that PAVN losses were 1,204 killed and 122 captured; American dead were 300 and South Vietnamese dead 100. The first two figures deserve the word claimed. They came from body counts: sweeps of jungle after the fact, in country where the other side carried away its dead, using a metric commanders were pressured to inflate and historians deeply distrust. Nobody can now say how many North Vietnamese soldiers died in the Ia Tchar Valley, only that most had walked south for months to reach it and that each was counted by his own army too. The 300 American dead were counted differently, name by name, and are not in dispute. When Francis Marion closed, the 1st Brigade went on screening the border. Two years later Westmoreland picked Peers to investigate My Lai; his 1970 report recommended charges against 34 people and became the most unsparing account the Army ever wrote about itself.