Quang Tri Province (the furthest north in South Vietnam) presented a number of obstacles to military planners. Political constraints limited US units from taking certain actions to counter or attack North Vietnamese forces. At different times during the war, restrictions existed on air attacks, naval gunfire, or ground movements into the DMZ or North Vietnam. Along the DMZ, the North Vietnamese forces in Quang Tri were closer to home than any place else in South Vietnam. Some of the more difficult battles of the war, such as that for Khe Sanh, were in this province and reflected some of the enemy's strengths there.
By 1967 the primary American combat unit in Quang Tri and Thua Thien Provinces was the 3d Marine Division, an outfit of 24,000 headquartered at Dong Ha with five infantry regiments, one artillery regiment, and supporting units. Most of the division, with the exception of the 4th Marines near Hue in Thua Thien Province, was positioned in a series of company-size strong points and battalion-size combat bases in northern Quang Tri Province to defend against a potential North Vietnamese thrust across the Demilitarized Zone. This barrier was known officially as the Strong Point Obstacle System but more commonly as the McNamara Line, named for Secretary of Defense Robert S. McNamara, who had been an early proponent of its construction. The McNamara Line employed thousands of acoustic and motion sensors (some dropped into Laos to monitor the Ho Chi Minh Trail) to give the allies data about enemy infiltration.
Given the emphasis on the northern battlefield, the Marines at the direction of General Westmoreland in April 1967 began the erection of the strong point obstacle system (SPOS) along the DMZ to prevent North Vietnamese infiltration. This so called "barrier" was to consist of three parts: (1) a linear manned obstacle system in the eastern DMZ sector extending some 34 kilometers to the sea and consisting of barbed wire, a 600 meter wide cleared trace, minefields, and electronic and acoustic sensors; (2) a series of strong points to the Laotian border built along obvious avenues of approach from the north with Khe Sanh as the western anchor; and (3) in Laos, the seeding of suspected infiltration routes with sensors monitored and supported by aircraft. The decision to build the McNamara Line or Electric Fence was prompted by the recognition that the American bombing campaign against North Vietnam had failed to stem the infiltration of men and materiel into the South. Although the name is usually associated with an antipersonnel barrier that was to span the Vietnamese isthmus just south of the demilitarized zone, it was actually only half of a complex multibarrier system. The second half involved an anti-vehicular barrier over the Laotian panhandle to interdict traffic on the Ho Chi Minh Trail.
Development of an unattended ground sensor system was begun during the Vietnam War in 1966 by Secretary of Defense Robert McNamara. In the SEACORE project, under DARPA auspices, United States Army Electronics Command (ECOM) scientists developed a number of electronic sensing devices, originally intended for use in the McNamara Line. These included sensors that could be emplaced by artillery, listening devices, and seismic detectors. Some, examples of which are on display in the Communications-Electronics Museum, were cleverly disguised as dog turds.
The anti-infiltration system was envisioned as a 40-kilometer-long physical barrier supported by early warning devices and carefully selected fortified positions constructed on key terrain and manned as appropriate. It was intended to counter the massive infiltration of North Vietnamese troops and equipment across the demilitarized zone.
It would have resembled the Morice and Pedron Lines the French built in Algeria - a cleared strip 600 to 1,000 yards wide, filled with barbed wire, minefields, and sensors, overseen by watchtowers, and backed up by a series of manned strongpoints and fire support bases. Like the Morice and Pedron Lines, the McNamara Line was characterized by physical measures (barriers, outposts, and reinforcing bases), but it was also supposed to have sensors as part of the barrier array.
The sensors, early warning devices, were designed to be used in a linear obstacle field. The balance pressure system would indicate any increase in weight, such as that produced by a person walking over it. The infrared intrusion detector operated in a manner similar to burglar detectors or an electric eye used to open the door of many business establishments. Unattended seismic detectors were used to note and report earth vibrations such as those caused by a group of men walking down a trail. Acoustic sensors transmitted the sound when men stepped on small explosive devices.
The majority of the firepower supporting the system came from artillery, tactical air fire, and naval gunfire. The system was designed to reduce the need for costly operations in an area constantly subjected to enemy-directed artillery and mortar fire from adjacent sanctuaries. The marines also hoped to use the anti-infiltration system to detect enemy incursions and movements at greater ranges. The system was an overall effort to counter both enemy infiltration and direct invasion by making enemy movement across the demilitarized zone simultaneously more expensive for the attacker and less expensive for the defender.
Work on the antipersonnel portion began in April 1967, at which time it was planned to be completed a year later. Its code name was Dye Marker, but McNamara Line stuck. Things turned out very differently, however. Preliminary work on the barrier was completed during the first few months of construction in 1967, but progress was slow, not in the least because the U.S. Marine Corps construction crews had to work within range of North Vietnamese artillery. Due to manufacturing problems, the sensor portion of the barrier system was never emplaced, thus creating holes in the Line's detection capability. The intensity of the enemy's mortar, artillery, and rocket fire and the changing military situation slowed down the development of the strong points and the construction of obstacles.
Strong enemy opposition and shortages of men and material slowed the progress of the SPOS. By mid September the 3d Marine Division had only completed the clearing of the trace from Con Thien to Gio Linh, a distance of 13 kilometers. Faced with mounting casualties, General Westmoreland approved a modification to his original plans. In essence, the division was to halt all construction of the trace until "after the tactical situation had stabilized," and continue only with the work on the strong points and base areas.
By the end of 1967, the Marines had completed work on the four strong points and all but two of the base areas. In the western sector of the barrier, only the base at Khe Sanh existed. By the end of December, 1967, the enemy appeared to be ready to make a fresh assault in northwestern South Vietnam at Khe Sanh. Following a period of relative calm since the battles earlier that spring near this isolated Marine base, American intelligence picked up reports of North Vietnamese troop movements in the sector. Although experiencing only limited combat activity at Khe Sanh in December, one Marine company commander declared that he could "smell" the enemy out there.
Then, the Tet offensive happened in the spring of 1968; sensors and other equipment destined for Dye Marker were diverted to the Marine defenders at Khe Sanh, where it proved to be extremely effective. Khesanh was a fortress constructed to command a major North Vietnamese supply route. It was besieged by NVA troops during 21 January-8 April 1968, but US forces, despite having to be supplied by air during part of the siege, successfully repelled every NVA attempt to take the fortress. When the siege was over, work on the McNamara Line was never resumed. The project was shelved when the buildup of U.S. forces in I Corps pre-empted the logistical support needed to supply the construction material.
The success of the Morice Line highlights the lesson that static barrage systems alone are a partial and temporary palliative at best inpreventing the opponent from moving toand from his area of operations. Successful barriers, whether at sea or on land, have involved static obstructions complemented with frequent and mobile patrols on theground. The Morice Line was manned by an average of 400 soldiers a mile. Had thisnumber been applied to Dye Marker, some 64,000 Soldiers and Marines would have been needed. Those numbers were not available; thus, if the McNamara Line had been completed as first envisaged, chances are that it would have leaked like a sieve.
General Westmoreland chose to employ a "strongpoint obstacle system" south of the DMZ. By his description, these strong points were: "fire-support and patrol bases, designed to channel the enemy into well-defined corridors where [the US] might bring air and artillery to bear and then hit jhe enemy forces with mobile ground reserves." In practice, this meant not defending on any type of line, but rather holding key positions and counter-attacking when targets were identified. In many respects, US operations in this area were no different from any other part of Vietnam. Units maintained secure bases, contributed to the local pacification effort, and fought with main force units when they were identified.
Although the barrier was never completed, certain portions of it were sufficiently developed to permit their use. The defense positions along the line were turned over to the Vietnamese Army, thereby freeing the American troops for mobile operations. Some of the early warning devices later used during the siege of Khe Sanh were reported to be effective. The information derived from the sensors provided targeting data for bombing and artillery strikes. While alone no deterrent to enemy movement, they provided a portion of the information that enabled friendly forces to bring the enemy under fire. The monitoring devices could be dropped from planes and helicopters, hung in trees, placed along river banks, or buried underground.
In 1970 Congressional hearings were called to discuss the value of the instruments. They were credited with saving many lives by giving ground troops early warnings of attack. The sensors were credited further with increasing enemy personnel and equipment losses as well as providing the Army with combat surveillance that worked by day or night. They were said to improve the effectiveness of air interdiction of enemy truck-borne troops and supplies and to conserve manpower. Some troop commanders stated that the use of the sensors permitted them to fight a major battle without the presence of their men. The sensors contributed to what was a difficult and, possibly the most important, task: finding the enemy and keeping track of his movements.
A Rand Corporation study was conducted in 1971 during the withdrawal of the US from Vietnam, when infiltration continued to be a problem. One proposed solution studied by the Rand Corporation was a barrier defense, not through Laos, but along the entire border of South Vietnam. It described how a force could have been established to block the type of threat posed by the North Vietnamese. The analysis indicated that the US could have built a defensive system which would have prevented infiltration.The report studied a linear defensive system, "Linear" meaning that forces would have been in fixed and permanent sites along a defensive line. There are otherways in which the US might have chosen to employ forces, a completely mobile defense being one of the most obvious.
The defensive line proposed in the Rand report was based on battalion strong points behind a linear obstacle belt consisting of sensors, mines, wire, a berm, and otherobstacles. The obstacle belt would have been approximately 150-300 meters wide. Heavily fortified blockhouses would monitor the belt and the main battalion strong point would have been located somewhat to the rear. These positions would have been well supported by artillery and aviation assets. Mobile reserve forces would also have been available to counter any attempt at large scalepenetration.
The Rand report contains a great deal of analysis of the rationale for, construction of, costs, andjustification for this program. The most important element was that in operation this system would require approximately one battalion on line per ten miles of front. This was mostly based on the idea that battalion base camps, located behind the actual line, would contain 155mm artillery. In the proposed configuration, a base camp every ten miles would ensure artillery support along the entire front. Since the DMZ-Tchepone Corridor Line was approximately 170 miles long, this would require seventeen battalions on line. Allowing one battalion to the rear for every two on line (for local reserves,rotation of forces, etc. . .), this would require a total of approximately twenty-seven battalions. This equals three divisions.
Assuming that an effective counter-insurgency campaign was continued in South Vietnam (continuing at the 1969 level), the report predicted that with an eighty-five percent effective barrier, enemy personnel strength in the south would approach zero in 4.2 years. However, if the enemy could cut his average losses in South Vietnam in half, the report indicated it would take close to 10 years to achieve these results. The time would also increase if North Vietnam chose to increase its infiltration attempts.
The Ho Chi Minh Trail through Laos was only one of several LOCs for the North Vietnamese. The Cambodian LOC was also important and gave North Vietnam until 1970 an equal capability for moving supplies and men. Cutting the Tchepone Corridor would have therefore caused great inconvenience but would not of itself have been a decisive factor in NVA and VC logistics.
Although "defend along a line" scenarios have their own frustrations, it might have been better than the types of operations being conducted in Vietnam. Tactical operations during the war were frustrating and measures of success were difficult to define. But Andrew Krepinevich, Jr., argues againstthe feasibility of an anti-infiltration force. His main points in opposition are that an end run through Thailand would have flanked the defensive line, and actual wartime experience alongthe DMZ showed how difficult such a fight might have been.
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