written by Todd DePastino

Man at podium pointing to map of Vietnam to illustrate McNamara’s Line


“McNamara’s Line” was an ambitious attempt to prevent North Vietnamese infiltration into South Vietnam and Laos across the DMZ, which sat roughly along the 17th Parallel.

Several military leaders endorsed the idea of defensive line across the DMZ almost from the time the border was created by the Geneva Accords in 1954, which divided Vietnam into two temporary “re-groupment zones”: the Communist North led by Ho Chi Minh and the American-aligned South led eventually by Ngo Dinh Diem and his successors.

General William Westmoreland, head of US forces in Vietnam, for example, thought a gigantic cordon of mines, barbed wire, guard towers, and masses of troops permanently stationed on the southern side of the DMZ would be a good way to go.

The Pentagon, instead, chose bombing infiltration routes, what Americans collectively referred to as the “Ho Chi Minh Trail.” The bombing was sanitized with the term “interdiction.”

By 1966, CIA and military intelligence assessed interdiction a failure. Operation Rolling Thunder and other bombing campaigns had little impact on North Vietnam’s ability to conduct military operations in the South. More troops would be needed, and more aggressive action on the ground to make the North cease and desist in its support of Viet Long insurgency.

The problem was that invading ostensibly neutral Laos and Cambodia and even North Vietnam itself was politically touchy. International condemnation would inevitably follow, as would the danger of widening the war and inviting more robust Soviet intervention.

To avoid over-escalating, Secretary of Defense Robert McNamara turned back to the idea of a defensive barrier.

Ever the Modernist manager and believer in technological innovation, McNamara proposed a “high tech barrier” to block infiltration down the Ho Chi Minh Trail in Laos and across the DMZ. McNamara turned this proposal over to JASON, a group of top scientists who advised the US government on military matters.

JASON issued its report to McNamara in August 1966. The scientists suggested the creation of two defensive barriers. The first would be an anti-personnel line running from the South China Sea coast in the east to the Laotian border in the west juts south of the DMZ. The second would be a more complex system west of the Laotian border and covering the 10,000 miles of networked trails, paths, and roads making up the Ho Chi Minh Trail.

1967 map of Vietnam and Laos showing McNamara’s Line

Jacob Van Staaveren: Interdiction in Southern Laos. Washington DC: Center of Air Force History, 1993.

The problem of the Ho Chi Minh Trail–and what made it at once such an unlikely and successful location for a supply route–was its remoteness, rugged terrain, and triple canopy primeval rainforest that hid activities from aerial surveillance.

Therefore, JASON called for this second defensive line in the mountains to be strewn with electronic detection devices that didn’t require troops on the ground.

The devices, in truth, already existed. The Navy had used “acoubouy” listening devices for submarine detection. The Air Force adapted the acoubouy to be dropped by parachute and hang in the tree canopy. Once in place, the sensor would detect noises over a certain decibel. The noises would be sent by radio signal back to a command center, where listeners would determine the noise source, most likely trucks and voices.

Just in case the enemy whispered and used mufflers on their trucks, areas where the acoustic sensors had been sewn would also be sprinkled with thousands of “button bomblets,” noisemakers that popped loudly when stepped on or disturbed. Once the noise source was confirmed, either by button bomblet or truck rumble, strike aircraft would be sent to the sensor’s coordinates to take out the enemy.

Some places on the Trail didn’t have thick tree canopies. In those locations, helicopters and airplanes would drop an “Air-Delivered Seismic Intrusion Detector (ADSID). The rocket-shaped devise would bury itself in the ground and then transmit evidence of vibrations made by troops and trucks.

ADSID III electronic seismic sensor

ADSID III electronic seismic sensor, one of several types dropped by U.S. aircraft along enemy roads and pathways in Southeast Asia. (U.S. Air Force photo)

Construction of “McNamara’s Line,” as soldiers, sailors, airmen, and Marines referred to it, began in 1967 and required a much infrastructure beyond the fancy sensors.

First, an ever-circling fleet of aircraft–mostly EC-121R “Batcats” and the Beech QU-22Bs–monitored the sensors and relayed signals they received back to a massive air-conditioned “Infiltration Surveillance Center” (ISC) in Thailand at Nakhon Phanom Air Base (NKP).

1970 photo of US military’s Infiltration Surveillance Center in Thailand

The US military’s Infiltration Surveillance Center in Thailand with an IBM 360/65 computer circa 1970 (Air Force Historical Research Agency)

The ISC had to be massive and air-conditioned because it housed two state-of-the-art IBM 360-65 mainframe computers that technicians used to process and interpret the data coming in from the circling aircraft.

Analysts at NKP would then report their findings to combat air commanders, who would then dispatch aircraft to strike.

The cost to run all this was close to $1 billion per year, not including ongoing research and development nor the NKP command center.

The Tet Offensive of early 1968 coupled with the simultaneous siege of Khe Sanh cut short the construction of the first barrier line across the DMZ Parallel and diverted much of the acoustic and seismic sensors to the hills surrounded the besieged Marine Combat Base at Khe Sanh.

The duel crises of Tet and Khe Sanh meant that McNamara’s Line was never brought up to its full strength. Then again, the line that did get established didn’t boast strong results.

One scientist, Robert S. Greeley, who worked on the project, remembers the the first acoustic sensor detected on the Trail. Voices of two NVA soldiers could be heard asking, “what’s this?” as they plucked a sensor from a tree and threw it in the back of their truck for later study.

That inauspicious start signaled the North Vietnamese’s quick response to the new technologies. No matter how the system was tweaked or its efficiencies improved, the bottom line–the movement of men and materiel from North to South–never stopped nor slowed.

‘There was precious little evidence that the system was having any effect,” recalls Greeley. “I found three specific occasions where our sensors indicated a target, the forward air controller actually saw it, an attack aircraft was called in, and an attack was made. In two of the attacks there were no secondary explosions or fires—the targets were said to have been killed but may just have disappeared under the jungle canopy. In the third attack, the target was a motorcycle, which the attacking pilot noted ‘turned around and sped off.’ . . . It all seemed anticlimactic, a technical and military failure and a huge waste of money and effort.”

Ultimately, McNamara’s Line was not able to fulfill its intended purpose. Despite the application of advanced technology and innovative ideas, it faced insurmountable challenges, leading to its inability to effectively disrupt North Vietnamese infiltration.

McNamara’s Line stands as an object lesson on the limits of technology in the face of a determined enemy.