The Secret Sentry (17 page)

But getting better at finding the enemy was just one of NSA’s big successes that year. After months of dissecting intercepted
North Vietnamese and Viet Cong radio traffic, in early 1966 NSA SIGINT analysts figured out that prior to every enemy attack,
the North Vietnamese and Viet Cong radio operators made significant changes to their transmitting procedures, including changing
their radio frequencies, cipher systems, and call signs, as well as establishing special backup radio centers and forward
command centers that only appeared in North Vietnamese radio traffic just prior to attacks. Radio traffic volumes also shot
up dramatically, as did the number of high-precedence messages being sent and received. With this analytic breakthrough, the
SIGINT analysts could predict, sometimes weeks in advance, when and where the enemy intended to launch an offensive, which
units were going to participate in the attack, and even what their objectives were. It would prove to be a hugely important
development that would cost the North Vietnamese and Viet Cong forces dearly in the years that followed, as American combat
forces were able to parry the enemy blow and frustrate enemy commanders time after time.
¹⁶

For example, in March 1966 SIGINT detected radio transmitters associated with a high-level North Vietnamese command unit plus
intelligence units moving toward the cities of Pleiku and Kontum, in the Central Highlands, suggesting that the North Vietnamese
were gearing up for an attack on the cities. The U.S. Twenty-fifth Infantry Division was sent into the region to preempt the
attack, forcing the North Vietnamese units to retreat back to their base areas in Cambodia after two months of battle. Then
in June 1966, another radio transmitter belonging to a North Vietnamese high-level headquarters was detected approaching the
highlands city of Dak To. This time, units of the 101st Airborne Division were sent in to clear out the North Vietnamese,
who were forced to withdraw in July. In October 1966, SIGINT detected the arrival of the NVA 324B Division in Quang Tri Province,
south of the DMZ. By November, elements of the NVA 341st Division had crossed the DMZ into Quang Tri. The North Vietnamese
intended either to launch a major offensive or to create a stronghold in the region south of the DMZ. In the battle that followed,
U.S. Marine units badly mauled the North Vietnamese division with the help of massive B-52 Arc Light air strikes.
¹⁷

As exemplified by the above, SIGINT proved to be instrumental in foiling virtually every North Vietnamese offensive during
1966 and in the years that followed, with some notable exceptions, such as the 1968 Tet Offensive, which is discussed later
in this chapter. The North Vietnamese offensive efforts in 1966 resulted in no tangible ground gains, but yielded massive
casualties among their troops. One has to wonder if the North Vietnamese military leadership ever stopped to question how
the Americans always seemed to know what their plans were. This may also have been the high point of the American SIGINT effort
in Vietnam.

Pound Them into the Dirt

For NSA, the year 1967 was marked by one resounding success after another on the Vietnamese battlefield. In April, SIGINT
detected a large North Vietnamese troop buildup in northern Quang Tri Province, south of the DMZ, with radio intercepts confirming
that the entire North Vietnamese 325C Division had moved into the region. Other data appearing in SIGINT indicated that the
NVA intended to launch an offensive to liberate Quang Tri and neighboring Thua Thien Province as early as June. Guided to
their targets with unerring accuracy by NSA information, B-52 bombers and navy and air force fighter-bombers smashed the North
Vietnamese troop buildup. The bombers were followed by a large force of U.S. Marine Corps infantry backed by tanks, artillery,
and air support. The 325C Division was for all intents and purposes wiped out as an effective military unit in the fighting.
¹⁸

Beginning in September, SIGINT detected another dramatic increase in the number of North Vietnamese radio transmitters operating
along the DMZ and in the A Shau Valley, just to the south. New North Vietnamese combat units were quickly identified in the
area south of the DMZ by SIGINT. This material, when matched with captured documents and information received from POWs and
defectors, led intelligence analysts in Washington to conclude that rates of North Vietnamese infiltration into these two
areas had reached invasion levels. The State Department’s intelligence staff issued a highly classified report warning that
SIGINT showed that four new North Vietnamese regiments had just arrived, or were about to arrive, in the area just south of
the DMZ. But MACV refused to accept the presence of thesenew units because, once again, the SIGINT data had not been confirmed
by captured documents or by prisoners.
¹⁹

Despite the nagging doubts of General William Westmoreland’s intelligence chief, General Phillip Davidson, about the validity
of much of the intelligence data he was getting from Fort Meade, SIGINT continued to rack up more impressive successes. In
October, SIGINT collected by the U.S. Army listening post in Pleiku revealed that the North Vietnamese First Division had
just crossed into South Vietnam from Laos and had massed near Dak To, a key garrison located northwest of Pleiku. In late
October, an accumulation of radio intercepts showed that an attack on Dak To was imminent, as evidenced by a dramatic surge
in the volume of North Vietnamese radio transmissions coming from the Dak To area from normal twice-a-day contacts to once
an hour. On November 1, elements of the U.S. Fourth Infantry Division and the 173rd Airborne Brigade were moved to Dak To
so as to preempt the anticipated North Vietnamese attack. The enemy offensive began on November 7. The bat-tle raged for ten
days, after which the battered First NVA Division broke off the engagement and retreated into Cambodia. The casualty counts
on both sides were massive, with 280 American paratroopers killed and 500 wounded in the battle. No one knows for sure how
many North Vietnamese soldiers were killed or wounded, but MACV estimated that 2,100 North Vietnamese were killed.
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The Battle of Dak To was considered by many senior American military commanders in Vietnam to have been SIGINT’s brightest-shining
moment up until that point in the war. But it was almost instantly eclipsed by an even more significant cryptologic breakthrough.

The “Vinh Window”

In October 1967, while the Battle of Dak To was still raging, radio intercept operators aboard a U.S. Air Force C-130 SIGINT
aircraft orbiting over the Gulf of Tonkin intercepted a new North Vietnamese radio net carrying what seemed to be routine
voice communications. The intercept tapes were brought back to the U.S. Army listening post at Phu Bai, where Vietnamese linguists
pored over them. Their analysis of the tapes showed that the North Vietnamese radio operators were passing mundane information
concerning low-level logistical matters over a newly constructed microwave radio-relay system linking the North Vietnamese
coastal cities of Thanh Hoa and Vinh. Situated just above the DMZ, Vinh was the location of a huge North Vietnamese logistics
center supplying the entire Ho Chi Minh Trail. From that point onward, C-130 SIGINT aircraft began regularly flying orbits
off the North Vietnamese coast targeting these en clair radio transmissions. Then in November, the nature of the traffic being
carried on this radio net changed, with intercepts revealing that the North Vietnamese radio operators were now sending complete
rundowns on the number of infiltration groups about to be sent down the Ho Chi Minh Trail from the Vinh base area. It was
an incredible find. NSA’s analysts now could determine how many NVA infiltration packets were traversing the Ho Chi Minh Trail,
as well as the size of the infiltration groups and their destination inside South Vietnam. In short, what NSA called the “Vinh
Window” appeared to be an intelligence bonanza of unprecedented proportions.
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President Lyndon Johnson and his national security advisor Walt Rostow were euphoric when they were briefed about the breakthrough
by NSA officials in early 1968. Everyone from the president on down suddenly believed that at last the United States could
attack the North Vietnamese infiltration route down the Ho Chi Minh Trail. A declassified NSA history states, “At the White
House, there was a sense that this intelligence breakthrough was the key [to the strategy of stopping infiltration].”
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But sadly, the Vinh Window ultimately proved in many respects to be a bust. NSA oversold the value of this SIGINT product
to its customers, promising them that the agency would be able to give them exact locations for the North Vietnamese infiltration
groups moving down the Ho Chi Minh Trail. NSA’s air force and navy customers complained when the agency was unable to produce
this kind of intelligence from the intercepts. In addition, the thousands of hours of intercepted North Vietnamese voice traffic
produced every month by American SIGINT reconnaissance aircraft orbiting over Laos and the Gulf of Tonkin swamped NSA’s small
cadre of Vietnamese linguists, and a proposal to use South Vietnamese personnel to transcribe the tapes was rejected for security
reasons. As a result, a massive backlog of hundreds of Vinh Window intercept tapes quickly built up, which, by the time they
were finally transcribed, analyzed, and reported, were already obsolete. As a declassified NSA history puts it, “What ever
tactical advantage that could have been gotten from the exploitation of the GDRS voice communications would never be realized.
Like the proverbial children at the candy store, American intelligence could only press its face against the Vinh Window and
imagine the oppor-tunity . . . the true goodies remained beyond our touch.”
²³

A Victim of Its Own Success

Despite the widespread disappointment that the Vinh Window intercepts did not allow the U.S. military to shut down the Ho
Chi Minh Trail, by the end of 1967 NSA had become a superstar, albeit a secret one, in Vietnam. U.S. military field commanders
in Southeast Asia were gushing in their praise of SIGINT. General Bruce Palmer Jr., the army’s vice chief of staff, told a
gathering of senior officers that SIGINT was for his commanders in Vietnam “the backbone of their intelligence effort. They
could not live or fight without it.” Palmer was not overstating the case. Declassified documents reveal that SIGINT was the
primary driver of U.S. Army combat operations in Vietnam, providing anywhere from 40 to 90 percent of the intelligence available
to U.S. forces about the strength and capabilities of the enemy forces facing them. Over half of all major U.S. Army offensive
operations launched in 1965 and 1966 had been triggered by intelligence coming from SIGINT.
²⁴

With each new success, senior army commanders in Vietnam became in-creasingly enamored of this seemingly magical fount of
knowledge, and in the process cast aside the more conventional sources of intelligence, such as POW interrogations and agent
operations. The result was that by 1967 dependence on SIGINT was so high that an American intelligence officer who served
in Vietnam told a congressional committee that American military commanders in Vietnam were “getting SIGINT with their orange
juice every morning and have now come to expect it everywhere.”
²⁵

But hidden behind the scenes, a tide of discontent was rising within the U.S. military and intelligence communities regarding
this source, among both those officials who had access to the material and those who did not. Meanwhile, there was also a
rising tide of antiwar sentiment in the United States, creating an increasingly intractable problem for the Johnson administration.
By 1966, public opinion had begun to turn against the war, even though the military continued to insist that the United States
was winning. Army and marine casualties were mounting, and by the end of 1966 almost five hundred American aircraft had been
lost and hundreds of pilots and crew killed or captured and held as POWs under terrible conditions. The next year saw an increase
in public demonstrations against the war and less than 50 percent of Americans supporting the way the war was being conducted.
Time was not working in favor of Johnson. Nevertheless, he continued to believe what he heard from his top commander in Vietnam,
General Westmoreland. Apart from the metric of body count, the military increasingly depended on various forms of intelligence—
above all SIGINT—to know whether or not the United States really was winning, and to anticipate and counter relentless enemy
pressure, from both the VC and the NVA.

Among the select few senior U.S. government officials and top American commanders with unfettered access to SIGINT, many were
worried that the U.S. military in Vietnam had become far too dependent on SIGINT. General Palmer, who valued it so highly
at the time, years later wrote that by 1968 MACV was largely reliant on SIGINT as its primary source of intelligence on enemy
movements and activities, and consequently placed less importance on HUMINT, POWs, and captured documents.
²⁶
NSA historians generally agree with Palmer’s assessment; one writes, “SIGINT had only part of the picture, and intelligence
analysts relied too heavily on the single source. In hindsight, it is clear that too little attempt was made to flesh out
the rest of the picture through interrogations, captured documents, and the like. SIGINT became the victim of its own success.”
²⁷

SIGINT generated so much information that the overworked intelligence analysts in Washington and Saigon were buried by the
mass of intercepts being produced every day, and as time went by, it became increasingly difficult to ascertain what was important
and what was not. In addition, the military command bureaucracy in Southeast Asia was so dense and multilayered that critical
intelligence reporting oftentimes failed to make it from the SIGINT collection units in the field to the military commanders
they were supposed to support in a timely manner, or fashioned in such a way that it could be immediately acted upon by field
commanders.
²⁸