By the spring of 1966, Secretary of Defense Robert S. McNamara had
soured on the air campaign against North Vietnam.
The Operation Rolling Thunder air strikes, which had begun in March
1965, had not accomplished much. The strategic results might have
been better, except that US airmen were severely constrained in
where and what they could bomb.
|An airman gets ready to hurl
a seismic sensor from an HH-3 helicopter over Vietnam to form
part of the electronic infiltration barrier approved by Defense
The targets were selected, one by one, by the White House. Strikes
on major military targets around Hanoi and Haiphong were seldom
The campaign was a measured and limited air action
to signal US determination and commitment to Hanoi.
Administration officials did not want to provoke a wider war.
They feared that stronger use of force might lead to a confrontation
with the Soviet Union or China.
Weapons, supplies, and troops streamed south from North Vietnam,
along the Ho Chi Minh Trail. North Vietnams military activities
were not seriously interrupted.
McNamara was opposed to any increase in bombing of the North. In
his view, the United States was not at war with North Vietnam. He
saw the conflict as one of insurgency in the South. The US objective,
he said, was not to overthrow or destroy North Vietnam but rather
to stop its infiltration and aggression in the South.
Within six months of the start of Rolling Thunder, McNamara was
rethinking the role of the air campaign in the overall US effort.
He was looking for an alternative.
The idea that McNamara seized upon came from professor Roger Fisher
of Harvard Law School, a part-time consultant to McNamara and John
T. McNaughton, the assistant secretary of defense for international
In January 1966, Fisher wrote to his friend, McNaughton, to suggest
that a 160-mile barrier be built across Vietnam and Laos. It would
consist of minefields, barbed wire, ditches, and military
strong points flanked by a defoliated strip on each side.
If the barrier could stop the infiltration, the bombing of North
Vietnam could be stopped, too.
Six weeks later, McNaughton passed the idea on to McNamara with
only slight revisions.
Around the same time, McNamara was approached by several top scientists
from MIT and Harvard who envisioned possibilities for a technical
solution in Vietnam.
McNamara asked the military for comment. The Joint Chiefs of Staff
were against the whole thing, saying it would take six or seven
Army divisions to clear and secure the terrain and up to 48 months
to complete the barrier. Gen. William C. Westmoreland at Military
Assistance Command Vietnam, Adm. U.S. Grant Sharp at US Pacific
Command, and all of the other senior commanders in the Pacific were
||The most widely
used sensor for Igloo White was the air-delivered seismic detection
sensor, which detected ground vibrations caused by enemy troops
or vehicles. It was small and light, appearing much like the
McNamara went ahead anyway. He asked a group of university scientists
called the Jasons to undertake a summer study looking into the feasibility
of the proposal
The Jasons were organized in 1959 by the Institute for Defense
Analyses, a think tank near Washington, D.C. The Jasons
name was suggested by the wife of one of the founders, who thought
the groups original name, Sunrise, sounded like a shaver.
The summer study was conducted by 47 Jasons, augmented by 20 IDA
analysts. They presented their report in person to McNamara on Aug.
30. They said the bombing of North Vietnam had had no measurable
direct effect on Hanois ability to mount and support military
operations in the South.
The report proposed building an antipersonnel barrier across Vietnam
below the Demilitarized Zone and an antivehicular barrier, consisting
of mines and battery-powered sensors, along the Ho Chi Minh Trail
The construction of the air-supported barrier could be initiated
using currently available or nearly available components, with some
necessary modifications, and could perhaps be installed by a year
or so from go-ahead, the Jasons said.
US aircraft had been flying interdiction missions in the Laotian
panhandle since December 1964. Air Force and Navy airmen were forbidden
to strike at the sources of supply in North Vietnam, so they chased
down trucks, one by one, on the jungle trails.
The Starbird Task Force
McNamara sent the Jason study to the Joint Chiefs of Staff for
evaluation, but he did not wait for their response before setting
the project in motion. On Sept. 15, he appointed Army Lt. Gen. Alfred
D. Starbird, a research and engineering officer, to head Joint Task
Force 728 to develop the barrier.
Starbird was given a target date of Sept. 15, 1967, to achieve
initial operational capability. The date was later extended but
only to Nov. 1, 1967.
In a follow-up memo to President Lyndon B. Johnson in October 1966,
McNamara said that Rolling Thunder had neither affected infiltration
or cracked the morale of Hanoi.
McNamara proposed an infiltration barrier, which he said would
lie near the 17th parallelwould run from the sea, across the
neck of South Vietnam (choking off the new infiltration routes through
the DMZ), and across the trails in Laos.
McNamara went on, This interdiction system (at an approximate
cost of $1 billion) would comprise to the east a ground barrier
of fences, wire, sensors, artillery, aircraft, and mobile troops;
and to the westmainly in Laosan interdiction zone covered
by air-laid mines and bombing attacks pinpointed by air-laid acoustic
The barrier, McNamara said, would be persuasive evidence
... that our sole aim is to protect the South from the North.
|Nakhon Phanom, Thailand, was
home to the Igloo White Infiltration Surveillance Center. Security
was so strict at the ISC that access was limited room by room,
and project airmen did double duty as janitors.
Starbirds task force was given the cover name of Defense
Communications Planning Group, which was meant to sound as bland
and as nondescriptive as possible. The project was called Project
Practice Nine. The Vietnam portion was Dye Marker. The Laos segment
was Muscle Shoals, and the technology for it was Igloo White. Igloo
White would be the name by which the entire program would be best
MACV commander Westmoreland met with Starbird and concluded that
the plan was a noble idea but also highly theoretical.
In his memoir, A Soldier Reports, Westmoreland said, As any
experienced military man would know, the concept had a basic flaw
in that no fenceelectronic or otherwisewould be foolproof
without men to cover it by fire, which raised the specter of tying
down a battalion every mile or so in conventional defense.
The McNamara Line
In January 1967, the White House, in National Security Action Memorandum
No. 358, gave Project Nine the highest national priority
for expenditures and authorization.
The program was freewheeling and free-spending. It was a heady
atmosphere for the scientists. They went to the front of the line
for materials and services. Whatever they asked for, they got.
If DCPG said it needed 10,000 chocolate cream pies from the
Army by noon the next day, it would get them and without any questions,
said a project member quoted by Paul Dickson in The Electronic Battlefield.
Along with its more solid work, Project Nine generated several
exotic schemes that were soon discarded. One such notion was to
train pigeons to carry munitions, land on North Vietnamese trucks,
and explode on touchdown. Among other difficulties, the pigeons
couldnt tell a communist truck from a noncommunist one.
|Sensor signals were too weak
to reach the ISC directly, so aircraft, such as this EC-121Rcall
sign Batcatmonitored the sensor line 24 hours a day, relaying
data back to Nakhon Phanom.
Another bizarre idea was to develop sensors that resembled dog
excrement. It was canceled after it was learned that there were
no dogs on the Ho Chi Minh Trail.
The task proved to be more difficult than the Jasons had figured.
Existing technology was not sufficient. The project required further
development and engineering on the sensors, aerial delivery, monitoring
equipment, processing and display, and other equipment.
Sketchy stories about a barrier in Vietnam showed up in the press
in 1966 and 1967, but the Pentagon professed to have only a limited
interest in any such idea.
Meanwhile, McNamara was increasingly vocal in his criticism of
the air campaign. I dont believe that the bombing up
to the present has significantly reduced, nor any bombing that I
could contemplate in the future would significantly reduce, the
actual flow of men and material to the South, he said at a
Senate Armed Services Committee hearing in January 1967.
Work on the Dye Marker barrier in Vietnam began in the summer of
1967. The job fell to the Marines, in whose area of responsibility
in Quang Tri Province the barrier was supposed to be built. It did
not go well. The Marines, who disagreed vigorously with the concept,
had to divert to it troops and resources that they could not readily
McNamara revealed the project to the public at a press conference
on Sept. 7, 1967, where he announced that a plan was under way to
build an electronic barrier south of the DMZ. (In fact, the project
was nearly a year old and parts of it were almost complete.) Syndicated
columnists Rowland Evans and Robert Novak reported that the Air
Force and the Navy were particularly opposed.
The press conference stimulated several popular names for the barrier,
but the one that stuck was the McNamara Line.
In January 1968, the Dye Marker sensors and other equipment, intended
for deployment along the DMZ, were commandeered for the defense
of the Marine outpost at Khe Sanh. The sensors were very effective
in tracking the enemy at Khe Sanheven the Marines said sobut,
when the siege lifted in April, work on the barrier did not resume.
By then, McNamara was gone, having left office Feb. 29, 1968. The
demise of Dye Marker was not made formal and public until March
1969, when Secretary of Defense Melvin R. Laird announced the cancellation
of the barrier project, saying, It did not work out as expected.
All that remained of the McNamara Line was Igloo White. Ironically,
the part that survived belonged to the Air Force, the service that
had been most opposed to McNamaras idea to begin with.
|Using the EC-121R was too expensive
and put a large crew at risk, so USAF turned to the QU-22B Pave
Eagle, a Beech Model 36 Bonanza loaded with electronic gear
and flown by a single pilot. It was also called the Quacker.
The bombing halt of 1968 marked the end of Rolling Thunder and
a change in the direction of the war. The focus of the bombing effort
shifted to interdiction of the Ho Chi Minh Trail, and Igloo White,
newly operational, was there to help.
Igloo White consisted of three parts: the sensors, the orbiting
aircraft to relay the signal, and the Infiltration Surveillance
Center at Nakhon Phanom Air Base in Thailand.
The sensorsa network of some 20,000 of themwere planted
mostly by Navy and Air Force airplanes, although some of them were
placed by special operations ground forces. They were dropped in
strings of five or six to be sure that at least three sensors in
each string would survive and be activated. The sensors operated
on batteries, which ran down after a few weeks, so replacement sensors
had to be dropped.
Most of the sensors were either acoustic or seismic. There were
two kinds of acoustic sensors, both derived from the Navys
Sonobuoy, to which microphones and batteries were added. These sensors
could hear both vehicles and voices.
The Acoubuoy (36 inches long, 26 pounds) floated down by camouflaged
parachute and caught in the trees, where it hung to listen. The
Spikebuoy (66 inches long, 40 pounds) planted itself in the ground
like a lawn dart. Only the antenna, which looked like the stalks
of weeds, was left showing above ground.
The ADSID (Air-Delivered Seismic Intrusion Detector) sensed earth
motion to detect people and vehicles. It resembled the Spikebuoy,
except it was smaller and lighter (31 inches long, 25 pounds). It
was the most widely used sensor in the program.
The challenge for the seismic sensors (and for the analysts) was
not so much in detecting the people and the trucks as it was in
separating out the false alarms generated by wind, thunder, rain,
earth tremors, and animalsespecially frogs.
There were other kinds of sensors as well. One of them was the
people sniffer, which chemically sensed sweat and urine.
Batcats and Big Computers
The nerve center for Igloo White was located at Nakhon Phanom,
which the Americans called NKP, in eastern Thailand,
across the Mekong River from Laos.
The Infiltration Surveillance Center was the biggest building in
Southeast Asia. The project broke ground in July 1967 and was in
operation before the end of the year. It had several names (including
Dutch Mill and Operating Location No. 1),
but the most popular one was Task Force Alpha.
Security in the center was tight. The people who worked there were
limited in which rooms they could enter. The janitors were all off-duty
Task Force Alpha airmen in the grade of E-5 or above.
|Running near the 17th parallel,
the McNamara Line cut across the Ho Chi Minh Trail in Laos.
After McNamara left, the USAF-maintained Igloo White portion
aided in air interdiction of the Ho Chi Minh Trail.
About 400 Air Force people were assigned to Task Force Alpha, and
a brigadier general was in charge. In the operational chain, the
center reported to 7th/13th Air Force at Udorn AB, Thailand.
The darkened war room contained rows of scopes. Its walls featured
large situation displays. Two IBM 360-65 computersthe most
powerful then availablecollated and processed the sensor data
for use by the target analysts. The computers also contained extensive
electronic maps of the Ho Chi Minh Trail and knew the precise locations
of the sensors. When something tripped one of the sensors, the computers
knew instantly where it had happened.
The sensor signals were too weak to reach NKP directly, though,
so aircraft orbited the trail, 24 hours a day, monitoring the sensors
on their radio receivers and relaying the information to Task Force
From 1967 to 1970, the aircraft flying the orbits were EC-121Rs
from the 553rd Reconnaissance Wing at Korat AB, Thailand. The EC-121
was a variant of the Lockheed Constellation airliner. It had four
propeller-driven engines and three distinctive tail fins. The EC-121R
differed from Air Defense Commands airborne warning and control
version of the EC-121 in that the radomes had been removed and special
electronics and antennas had been installed.
The 553rd RWits call sign was Batcatflew
its first operational mission on Nov. 25, 1967. The EC-121R carried
a crew of 17 or 18, including a combat information center crew of
six or seven working the Igloo White gear in the back of the aircraft.
It was a long duty day. To provide round-the-clock coverage of
the sensors, the Batcats orbited the Ho Chi Minh Trail in eight-hour
shifts. With transit times, the flights lasted about 10 hours. Briefing
and debriefing made the missions still longer.
The 121s sent a live feed of the sensor alarm data to the computers
at Task Force Alpha. (Airmen working the scopes at NKP could not
listen directly to the sensor microphones, but the mission crews
on the aircraft could and did.) Input from the audio sensors was
also recorded and kept on tape.
For several reasons, the Batcat crews simultaneously tracked and
analyzed the sensor data they were relaying. Doing so maintained
continuity if the link to NKP was lost or in case a Task Force Alpha
computer or other piece of equipment crashed. In addition, several
parts of the trail were outside the range of the relay equipment.
At those locations, the aircrews processed the sensor signal data
manually and called it in to 7th Air Force.
The EC-121R did a good job, but it was expensive to operate and
it exposed a large crew to enemy fire. Consequently, it was replaced
by a smaller airplane.
The QU-22B Pave Eagle was a single-engine, propeller-driven aircraft,
a modified Beech Model 36 Bonanza, designed to fly in either a manned
or unmanned mode. All of the operational flights, however, had a
pilot. There was no room for anyone other than a single pilot. The
rest of the cabin was filled with electronic monitoring equipment.
The QU-22, sometimes called the Quacker, was based
at NKP, considerably closer to the trail than were the Batcats at
Korat. In September 1970, the 553rd RW commander certified that
Pave Eagle was performing satisfactorily and that it could accomplish
its combat objectives. The 553rd was deactivated in December and
the QU-22s took over.
The QU-22 program itself was canceled in 1972.
We wire the Ho Chi Minh Trail like a drugstore pinball machine,
and we plug it in every night, an Air Force officer told Armed
Forces Journal in 1971. Before, the enemy had two things going
for him. The sun went down every night, and he had trees to hide
under. Now he has nothing.
Seventh Air Force in Saigon, which had operational control of air
strikes on the Ho Chi Minh Trail, ruled that Igloo White would augment,
not replace, the other interdiction efforts. Strikes on the targets
developed by sensors were controlled by an airborne command and
control center, just as other strikes in Laos.
The sensors tracked the direction and speed of the truck convoys.
From those data, it was easy to figure out where the trucks were
going and when they were likely to get there. If circumstances warranted,
strike aircraft might be on the scene in five minutes. The sensors
continuously updated the location of the trucks as the strike aircraft
|Although USAFs AC-130
could find its own targets using onboard systems like Black
Crow, the gunship also made use of Igloo White data to
locate and strike enemy vehicles, such as those seen in this
image from an AC-130 strike.
At a Pentagon news briefing in February 1971, Brig. Gen. William
G. Evans, Air Force special assistant for sensor exploitation, explained
that the sensors were identifying a strike zone.
We are not bombing a precise point on the ground with a point-target
bomb, Evans said. We cant determine each trucks
location that accurately with ground sensors, which are listeningnot
viewingdevices. Since we never actually see the
trucks as point targets, we use area-type ordnance to cover the
zone we know the trucks to be in. ... [Thus] we have an interdiction
system which can hurt the enemy, even when he seeks the cover of
foliage, weather, or darkness.
The sensor data were also used to develop non-time-sensitive targets
such as truck parks and storage areas, which could be stuck at any
Many kinds of aircraft flew missions against the Ho Chi Minh Trail.
Among them were B-52s, B-57 light bombers, F-4s, and assorted Navy
However, the most effective weapon of all against trucks on the
trail was the AC-130 gunship. It had its own sensors, including
low-light-level TV, forward-looking infrared, and the shadowy Black
Crow, which could detect truck engines from 10 miles away.
For the most part, the AC-130s found their own targets, but they
also received and used information from Task Force Alpha.
The North Vietnamese were aware of the Igloo White sensors and
took countermeasures. They destroyed some and tried to induce false
reports by others. Among other techniques, the North Vietnamese
drove animals up the trail and hung buckets of urine in the trees
to foil the sensors.
The acoustic sensors picked up some memorable moments, all of which
were duly recorded by the EC-121 crews. In The Electronic Battlefield,
author Dickson described a tape recording that was played for him:
It contained a few critical minutes in a North Vietnamese
truck park along the Ho Chi Minh Trail in Laos and was recorded
during an Igloo White mission, Dickson wrote. The first
sound heard is that of a single truck parked with its engine running.
It can be heard for several minutes during which time the only other
sound one can make out is distant artillery.
Then a voice can be heard shouting excitedly. The first truck
is now moving and others are starting and some drivers are using
their horns. Suddenly, there is the unmistakable sound of a jet
zeroing in, followed by a quick series of sharp explosions and the
jet pulling away.
Save for the sound of a few of the surviving trucks which are
getting more and more distant, there is relative quiet for a few
moments and then comes the sudden and loud pockata-pockata of anti-aircraft
The Air Force played another tape for a Senate committee. The sound
of axes could be heard as an inept work crew chopped down a tree
to obtain a sensor caught in the branches overhead. That was followed
by the sound of a crash and screaming as the tree fell on them.
In another documented instance, a North Vietnamese NCO is heard
telling a trooper to climb a tree to get the parachute from an Acoubuoy
snagged in the foliage. He wanted to give the material to his girlfriend
to make a dress.
One limitation was that not every branch and offshoot of the trail
was wired. There were entire routes that Igloo White did not know
about. Even at the peak of the bombing, the North Vietnamese were
building one or two miles of new road a day. In 1972, North Vietnamese
tanks appeared in South Vietnam, having come all the way down the
trail without being detected, much less stopped.
In March 1970, President Richard M. Nixon publicly acknowledged
the US military involvement in Laos. As further details were disclosed,
Igloo White came under considerable scrutiny.
Doubts and Quips
Did it work? Unquestionably, the sensors detected tens of thousands
of trucks, and the strike aircraft destroyed many of them. Some
of the anecdotal evidence is spectacular. In one instance, the bombs
squarely hit a targeteither a fuel or ammunition storage areathat
had been found by the sensors, and the mushroom cloud from the secondary
explosion rose 8,000 feet into the air.
Seventh Air Force reported that, during the 1968-69 dry season,
6,000 trucks were destroyed or damaged on the Ho Chi Minh Trail.
(Traffic on the trail surged during the dry season, from November
to May, and slackened or stopped in the wet season, from June to
October.) The total reported for the 1969-70 dry season was 10,000,
and for 1970-71, it was 20,000.
The published data does not say how many of these trucks were destroyed
or damaged as the result of identification by the Igloo White sensors.
Not everybody believed 7th Air Forces numbers. For example,
the sarcasm fairly dripped from an April 1971 commentary prepared
by the staff of the Senate Foreign Relations Subcommittee on US
Security and Commitments Abroad.
These figures are not taken seriously by most US officials,
even Air Force officers, who generally apply something on the order
of a 30 percent discount factor, the staffers said. One
reason why there is some skepticism about the truck kills claimed
by the Air Force is the total figure for the last year greatly exceeds
the number of trucks believed by the embassy to be in all of North
The Central Intelligence Agency chimed in, saying that 7th Air
Forces numbers game was refuted by the CIAs
own highly reliable sources, referring to its agents
in the enemy ranks. The CIA and the Defense Intelligence Agency
developed a formula that arbitrarily discounted 75 percent of the
However, if Air Force claims could be cast into doubt, so could
the criticism. Intelligence data and CIA reports were not always
accurate. As for comments by Congressional staffers, politics might
be suspected to be a leading ingredient.
Then, as now, the bomb damage assessment process was flawed on
both ends: Operations tended to claim too much; Intelligence tended
to validate too little.
Two facts seem indisputable: The air strikes got a lot of trucks,
but a lot of other trucks got through. The bomb damage claims may
have been high, but perhaps not as high as the critics have alleged.
In a recent analysis, Air Force historian Eduard Mark has calculated
a rough correspondence between the number of trucks the North
Vietnamese imported from their allies and the number of those the
Air Force claims to have destroyed in Laos. This does not prove
the validity of the claims; but there is at least not the kind of
gross discrepancy that would discredit them prima facie. It is,
accordingly, not unreasonable to take the claims for trucks destroyed
as a basis for discussion.
The war entered a different phase in the spring of 1972, when North
Vietnam launched a major military invasion. The North Vietnamese
Army crossed the DMZ directly and in strength to challenge the South
Vietnamese ground forces, who were increasingly on their own.
After a four-year hiatus, the bombing of North Vietnam resumed
in Operations Linebacker I and Linebacker II. The weight of US air
strikes, which had been concentrated on the Ho Chi Minh Trail, shifted
to targets in North Vietnam. Troops and supplies were bombed north
of the DMZ, before they could move onto the trail.
After January 1972, the Igloo White sensors detected a big drop
in the number of trucks moving through Southern Laos.
US objectives had also changed. A major goal of the bombing was
to push North Vietnam toward a negotiated settlement of the war.
In aid of that, the air strikes concentrated on Hanoi, Haiphong,
and military centers in the north.
Igloo White operations on the trail diminished in 1972 and then
stopped altogether. The computers at Nakhon Phanom were packed up
and shipped home to the United States.
There was, however, one last footnote. On March 29, 1973, 7th Air
Force left Saigon and moved to NKP. It set up headquarters in the
Task Force Alpha building, where it took on an additional role as
the US Support Activities Group. It operated there in that capacity
until it was deactivated on June 30, 1975.
John T. Correll was editor in chief of Air Force Magazine for 18 years and is now a contributing editor. His most recent article, “Into the Valley of Fire,” appeared in the October issue.
Copyright Air Force Association. All rights reserved.