The objective of those attacks was the logistical system of the People's Army of Vietnam (PAVN) that snaked through southeastern Laos and was known as the Ho Chi Minh Trail (the Truong Son Road to the North Vietnamese).

By July 1968, the munitions had proved to be relatively ineffective, and the use of sensors to obtain reconnaissance information was rapidly becoming the principal objective of the Igloo White system.

[3] After the initiation of the strategic aerial bombardment of North Vietnam (Operation Rolling Thunder) in March 1965, Washington saw that program as its chief method of relaying signals to Hanoi to cease its support of the southern insurgency.

In January McNamara was presented with a working paper by the American academic Roger Fisher, who proposed a less costly physical and electronic barrier that would be located in South Vietnam.

On 15 September 1966, McNamara made Army General Alfred D. Starbird the head of the newly formed Defense Communications Planning Group (DCPG), which was to oversee the implementation of the program.

[14] In June 1967 the barrier project, by then referred to as the Strong Point Obstacle System (or SPOS) was renamed Illinois City, which lasted for a month before the program was redesignated Dyemarker.

[15] In mid-1968, the physical barrier concept was pushed aside after the North Vietnamese overran the Lang Vei Special Forces camp and besieged Khe Sanh.

The first sensors utilized by the program were Air-Delivered Seismic Intrusion Detector (ADSID), which had been developed from devices then in use in underground mapping for the oil industry.

[19] The first acoustic sensors were developed from the U.S. Navy's Project Jezebel anti-submarine warfare sonobuoys, which recorded and processed sound by the utilization of an audio spectrum analyzer.

[20] The Phase I models of both the acoustic and seismic sensors were only available for operation in a continuous mode, which meant that under normal conditions, their lithium batteries would function for approximately 30 days.

These notably included the BLU-31/B and Mk 36 air-dropped mines, BLU-43/B and BLU-44/B landmine system (Dragontooth), the BLU-72/B fuel-air explosive (Pave Pat), and the BLU-52/A chemical bomb (filled with bulk CS-2 powder).

[15] PAVN personnel moving on foot through the trail system would be detected by the detonation of air-sown, aspirin-sized, wide-area gravel mines, which would activate the sensors.

Subsequent bomb damage assessment missions and hand emplacement of sensors and mines in support of Muscle Shoals would be carried out by the reconnaissance teams of the highly classified Military Assistance Command, Vietnam Studies and Observations Group (SOG).

At Task Force Alpha, the last of the three components in the system, the intelligence data (from a variety of sources, not just the sensors) would be entered, collated, retrieved, and stored by two IBM 360/40 (later two 360/65) computers.

The computers analyzed sensor data and compiled intelligence information and then made predictions as to where and when a particular PAVN truck convoy would be geographically located.

Due to increasing PAVN anti-aircraft artillery defences encountered in southeastern Laos, delivery in high-risk areas of the trail system was handed over from the Neptunes to Air Force F-4 Phantom II fighter-bombers that had been specially equipped for the missions.

The EC-121Rs and their crews proved too vulnerable and were partly replaced in 1969 and 1970 by QU-22Bs (modified Beech A-36 Bonanzas) which were to be remotely piloted,[34] and which had undergone primary mission equipment and PME flight tests at Eglin Air Force Base, Florida, in 1968.

[37] This process was stopped by the battle of Khe Sanh, when an estimated three PAVN divisions approached, and then surrounded the Marine outpost in western Quang Tri Province adjacent to Laos.

The U.S. commander in Vietnam, General William C. Westmoreland, ordered Task Force Alpha to support the aerial effort to defend the base (Operation Niagara).

[38] Although the Marine Direct Air Support Center was at first reluctant to trust the sensors (which in fact replaced ground patrolling), they were soon convinced of their utility.

[44] This intelligence was delivered from Nakhon Phanom to the Seventh's headquarters at Ton Son Nhut Air Base, where it issued the strike orders.

The only exception to this arrangement was Operation Commando Bolt, a real-time, LORAN-based technique which utilized sensor strike zones derived from predicted target locations.

[45] These missions were coordinated by Task Force Alpha's Sycamore control center against targets that passed through strike modules (four strings of three to six sensors each).

The SPOS outlined in the original Dyemarker plan envisioned a virtual defensive wall stretching from the South China Sea to Dong Ha Mountain.

[53] On 29 October 1968, work was stopped on the physical SPOS barrier, by that time redesignated as Duel Blade, under MACV Planning Directive 10–67, citing changes in the overall force posture, both enemy and friendly.

The physical barrier would be replaced by the revised Duel Blade plan of active resistance utilizing air strikes, artillery and naval bombardment and mobile reaction forces.

The report went on to state, however, that the lapse was not the result any failure of the sensors themselves, but the Air Force's false assumption that the trail net had adequate coverage.

[63] Thomas C. Thayer, chief of the Advanced Research Projects Agency for four years during the war, believed that only about one-twentieth of the cargo imported into the north moved southward on the Ho Chi Minh Trail and that more than two-thirds eventually reached the battlefields in the south.

[64] More recently however, a new study by Air Force historian Eduard Mark established a correlation between trucks imported into North Vietnam during the war and those that were claimed by the American pilots as destroyed.

[65] Access to communist Vietnamese archives to scholarly research may add to the true effectiveness (or lack thereof) of the American electronic and aerial effort.