During the late spring and early summer of 1966, Marine reconnaissance patrols operating south of the DMZ began filing unusual reports. The soldiers they were encountering were not the irregular guerrillas of the Viet Cong — they were uniformed regulars, moving in formation, with the equipment and discipline of a conventional army. The PAVN 324B Division had crossed the DMZ largely intact, and was massing in the jungle ridges and valleys between the river and Highway 9. Military leadership ordered a response. Task Force Delta, built around multiple Marine battalions, was assembled at Dong Ha and pushed north and west into terrain that was brutally difficult for any force trying to operate with helicopters. The thick jungle canopy made visual navigation nearly impossible and helicopter landings a matter of carving clearings with chainsaws.

The fighting came to its sharpest point on July 24th, on a feature the Marines designated Hill 362, roughly 7.5 kilometers north of the Rockpile. Company I, 3rd Battalion, 5th Marines was setting up a radio relay station when the PAVN opened fire from concealed positions on the hillside. The 2nd Platoon was caught in the open and took heavy casualties. Lance Corporal Richard A. Pittman, a twenty-one-year-old Marine from the 1st Platoon, ran forward carrying a machine gun he had picked up from a fallen comrade and laid down suppressive fire that allowed the survivors of the 2nd Platoon to withdraw to the hilltop. The dead and wounded could not all be recovered. One survivor lay motionless among the dead as PAVN soldiers moved through the position. By dawn, Company I had suffered 18 dead and 82 wounded. Pittman was later awarded the Medal of Honor for his actions that day.

The terrain and weather conspired against the Marines almost as effectively as the PAVN. Typhoon Ora struck the operational area while the battle was ongoing, bringing heavy rains that turned slopes into mud, grounded aircraft, and complicated the evacuation of wounded soldiers. Only eleven wounded could be extracted from Hill 362 before conditions made further helicopter operations impossible; engineers had to be lowered by rope to cut a landing zone in the canopy. The combination of the thick jungle, the poor visibility, and the PAVN's ability to close to within five meters of American positions — near enough for Marines to hear them talking and breathing — meant that the overwhelming advantages of American firepower were repeatedly neutralized. The fighting in Helicopter Valley was the grinding, close-quarters combat that the DMZ region would produce again and again over the next two years.

Operation Hastings officially ended on August 3rd, 1966, though its last major action had come on July 28th, when a Marine reconnaissance patrol spotted 150 to 250 PAVN soldiers five kilometers southwest of the Rockpile and called in artillery strikes that killed at least 50. The 324B Division had been pushed back across the DMZ or dispersed into the western jungles. General Lewis Walt, commanding Marine forces in I Corps, summarized the enemy they had faced with a notable frankness: "We found them well-equipped, well-trained and aggressive to the point of fanaticism. They attacked in massed formations and died by the hundreds." Both the costs and the candor were genuine. It was a qualified success — the immediate threat had been stopped — but the conditions that produced it had not changed.

What Operation Hastings confirmed was that the DMZ would not function as its name implied. The PAVN had no intention of treating the zone as a barrier, and U.S. military leadership responded by ordering a steady buildup of Marine forces in northern I Corps that would continue from 1966 through 1968. The Stingray Patrol concept — small, fast reconnaissance teams that called in firepower rather than engaging directly — was born from the lessons of these operations, named by General Walt after the July 28th action. The men who fought in Helicopter Valley in the summer of 1966 were the first wave of what would become years of continuous conflict along this stretch of terrain. The valley retains its shape in satellite imagery today: steep ridges, deep canopy, the Thạch Hãn watershed draining it all toward the sea.