Ninety kilometres northwest of Đồng Hới, the Annamite Range thins to a notch. It is not a dramatic crossing — under 500 metres above sea level, modest for a range that runs the length of Indochina — but the approach to it is a trap. A CIA landscape analysis written in February 1966 described the road threading "its way upstream along a narrow, steep-sided valley. To the left rise dog-toothed limestone peaks, to the right is a flat-topped plateau. Dense tropical rain forest covers the entire area, almost frustrating aerial observation. The road is carved out of the steep hillside, for in most places there is not enough room for both road and stream in the constricted bottom of the ravine." On the Vietnamese side the highway climbs from Tân Ấp; beyond the crest it becomes Route 12, dropping into Khammouane Province in Laos. Analysts studying that valley saw exactly one thing worth seeing: a place where everything had to funnel.

By March 1966, American estimates put three-quarters of all truck traffic entering Laos through this single gap. That made Mụ Giạ the principal doorway into the Hồ Chí Minh trail, and the arithmetic of interdiction looked obvious. Barrel Roll had already brought aircraft over the region, Rolling Thunder was at its height, and Commando Hunt would follow. On 12 April 1966, twenty-nine B-52s struck the pass for the first time, in what was described at the time as the largest bombing mission since the Second World War. They used a mixture of subsurface and delayed-action bombs across a five-kilometre stretch, the intention being less to crater the roadbed than to bring the mountainsides down on top of it. The landslides never came. The trucks kept moving.

By 1967 the failure was plain enough that the Department of Defense went looking for something larger, and the question reached JASON. A weapon of that yield, the reasoning ran, could do what conventional ordnance had not: collapse the pass physically and permanently. The scientists advised against it, and their argument was about consequence rather than capability. If the United States used a nuclear weapon first, they wrote, China or the Soviet Union might well respond by supplying comparable weapons to North Vietnam and the Viet Cong. It is a strange document to sit with half a century on. A panel of physicists was asked to price the destruction of one valley, and answered by pointing out where the bill would actually be presented.

The People's Army of Vietnam answered the bombing by refusing to depend on any single line. Bypass roads were cut east and west of Route 12 on the Lao side, so that a severed section became an inconvenience rather than a stoppage. Early in 1969 a fuel pipeline crossed the frontier through the pass, moving petroleum underground where drums on open truck beds could be seen and hit. Behind all of it were people, most of them not soldiers in any conventional sense. As many as 43,000 North Vietnamese and Laotians worked the trail system at any one time — filling craters, rebuilding bridges, cutting fresh road through karst under a bombing campaign that never really paused. In 1969 alone, 433,000 tonnes of ordnance fell on Laos. Neither the United States Air Force nor the Navy ever put Mụ Giạ out of operation for any sustained period.

The pass is an ordinary working border crossing now, the Cha Lo gate, with National Highway 12A climbing roughly twenty kilometres over it from the Khe Ve junction. Trucks still labour up the grade, carrying timber and cement rather than ammunition. Forest has closed over most of the ground the bombs opened, though not over the ordnance itself, which is still being cleared on both sides of the line and will be for years yet. Something else is coming, too. The rail link the French abandoned in 1950 has been revived as the Vientiane–Vũng Áng project: a feasibility study funded in 2015, a letter of intent signed in April 2017, and in March 2026 a concession agreement for the 147-kilometre section running from Thakhek to Mụ Giạ. If it is built, freight will cross this notch on steel for the first time, roughly ninety years after someone first drew the line on a map. So far, the mountain has outlasted every plan made for it.