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The McNamara Line

In September 1967, U.S. Secretary of Defense Robert S. McNamara announced a plan for the construction of an electronic anti-infiltration barrier south of the DMZ in Vietnam. The stated purpose of the barrier was to “sound the alarm” when North Korean troops attempted to enter the protected area. Once detected, U.S. forces and its allies would respond with air strikes and artillery bombardment.

What a concept

Artificial barriers predate even the cultivation of plants. The Great Wall of China and Hadrian’s Wall in Great Britain were erected to block the tide of invading barbarian forces. Even Vietnam itself was home to two huge walls built by the Nguyen to separate themselves from the northern Trinh armies in the early 1600s.

The French considered constructing a barrier at the narrow part of Vietnam. After their defeat at Dien Bien Phu in 1954, the plan became irrelevant. The French did, however, construct a meaningful barrier in Algeria, called the Morice Line, in 1957. The Maginot Line was likewise constructed by the French prior to World War II.

The barrier concept in Vietnam was considered as early as 1958 by American advisory personnel. Another proposal was put forth in 1961 to create a “cordon sanitaire,” or quarantine line, along the South Vietnam-Laotian border to prevent infiltration by the North Vietnamese. The Southeast Asia Treaty Organization (SEATO) proposed a similar concept, manned by international forces, along the DMZ and the Laotian panhandle, in the same year. General William Westmoreland also backed a similar plan in 1964.

Those barrier proposals were put on the back burner because it was thought, by officials in Washington, that heavy bombing initiated by Operation Rolling Thunder would slow the infiltration.

According to The Pentagon Papers, bombing sorties numbered 55,000 in 1965 and increased to 148,000 in 1966. Bomb tonnage rose from 33,000 in 1965 to 128,000 and the number of aircraft lost rose from 171 to 318 with estimated costs totaling $1.2 billion in 1966.

As the realization dawned that the bombing policy was not having its desired affect, McNamara began to seek other options.

High-tech barrier

A plan was devised by Harvard Law School professor Roger Fisher to install a barrier of state-of-the-art electronic devices along the DMZ and the Ho Chi Minh Trail. The idea was turned over to the Jasons, an elite group of about 45 of the nation’s leading academic scientists. They reached the same conclusion on the bombing issue and expanded the infiltration barrier concept to include two components:

  • An antipersonnel barrier, manned by military personnel, spanning south of the DMZ from Laos to the South China Sea, a distance of about 160 miles. The antipersonnel barrier was to comprise minefields, ditches, barbed wire, and defoliated strips with military strongholds at specified, geographically advantageous positions.
  • An antivehicular barrier to interdict traffic on the Ho Chi Minh Trail. The barrier was to consist of numerous sensoring devices of various styles and applications, and monitored in Nakhon Phanom, Thailand.
  • Of operating acronyms and code names

    The McNamara Line was first given the code name “Project Nine.” MACV, U.S. Military Command, Vietnam, then changed the name of the plan to “Dye Marker,” following a compromise of the classified Project Nine sobriquet.

    At that time, September 1967, the North Vietnamese began Phase I of their “General Offensive, General Uprising” campaign by attacking marine positions along the DMZ. That made it especially difficult to advance the McNamara Line`s construction.

    As January 1968 came and went, NVA troops were massed for an all-out attack on the Marine base at Khe Sanh as part of the Tet Offensive. Sensors and hardware had to be diverted from other parts of the DMZ to Khe Sanh. After that siege ended in April, construction on the McNamara Line was abandoned.

    Interdicting the Ho Chi Minh Trail

    As bombing shifted in March 1968 from North Vietnam to the Ho Chi Minh Trail in Laos, the antivehicular barrier helped to increase the effectiveness of fighter-bomber sorties. The air portion of the mission to place the sensors along the trail was code-named “Muscle Shoals,” while the electronic interpretive technologies held the tag of “Igloo White.”

    The sensors were about 20,000 in all, either seismic or acoustic, some half-embedded in the ground, others dropped by parachute so they would hang in the trees. They came in three main types:

  • The “Acoubuoy,” 36 inches long and 26 pounds, were camouflaged and floated down by parachute;
  • the “Spikebuoy,” 66 inches long and 40 pounds, stuck in the ground like a lawn dart with the antenna camouflaged to resemble weeds;
  • and the ADSID (Air-Delivered Seismic Intrusion Detector), resembling a Spikebouy but smaller at 31 inches and 25 pounds — the most widely used sensor.
  • Other sensors included a “people sniffer,” designed to sense sweat and urine. The newly installed “Black Crow” detection system could sense truck engine emissions from 10 miles away.

    As Igloo White interpreted signals from the sensors, they sent out directives to guide newly developed gunships to their targets. The code name for those AC-130 gunships was “Pave Spectre.” They carried 40mm cannons capable of firing 6,000 rounds per minute.

    Igloo White was disbanded in 1972 because of high operational costs, and military officials thought a cease-fire was imminent.

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