The first battle of the new “American phase” in the Vietnam War began in August. Early that month, U.S. Army airborne radio direction finding (ARDF) aircraft flying routine SIGINT collection missions over the northern portion of South Vietnam picked up a heavy volume of Viet Cong Morse code radio messages coming from just south of the Marine Corps base at Chu Lai. By mid-August, the ARDF aircraft had discovered the source of the Morse code transmissions and the identity of the Viet Cong unit sending the messages. The transmitter belonged to the headquarters of the two-thousand-man First Viet Cong Regiment, which was secretly concentrating its forces on the Van Tuong Peninsula, fifteen miles south of Chu Lai. The information was fed to General Lewis Walt, commander of the Third Marine Amphibious Force, who immediately initiated a search-and-destroy operation against the VC regiment. Designated Operation Starlight, it commenced on August 18. A marine battalion quickly penned the VC regiment up against the sea, while another marine battalion landed on the peninsula and began wiping out the trapped Viet Cong forces. By August 24, the marines reported that they had destroyed two battalions of the VC regiment, killing an estimated seven hundred Viet Cong troops. On the negative side, over two hundred marines had been killed or wounded in the fierce fighting. Despite the heavy casualty toll, NSA officials considered the success of Operation Starlight to be SIGINT’s most important accomplishment in Vietnam up until that point.7
Unfortunately, as was too often the case during the war, the use of body count metrics to measure success during Operation Starlight produced a chimera. In fact, SIGINT showed that the majority of the First Viet Cong Regiment had somehow managed to escape from the Van Tuong Peninsula. According to a declassified NSA history, radio intercepts showed that “within two days of the battle, the First Regiment’s radio network was back on the air.”8
Two months later, in October, three regiments of the 325th NVA Division launched an offensive in the Central Highlands with the objective of cutting the country in half. In this first offensive in the south, NVA regulars scored a quick victory at the Plei Mei Special Forces camp, twenty-five miles south of the city of Pleiku, but then were forced to retreat up the nearby Ia Drang Valley when confronted by a strong force of American infantrymen belonging to the newly arrived First Cavalry Division (Airmobile), commanded by Major General Harry Kinnard.
As the 325th NVA Division retreated deeper into the Ia Drang Valley, it was shadowed by five ARDF aircraft tracking the locations of the radio signals of the division’s commander and his subordinate regimental commanders, which enabled Kinnard’s forces to leapfrog up the valley in their Huey he licopters, harrying the retreating division every chance they got. At about four thirty a.m. on November 14, a tactical SIGINT intercept team attached to the First Battalion, Seventh Cavalry, intercepted a transmission indicating that a battalion of the 325th NVA Division (the Ninth Infantry Battalion of the Sixty-sixth NVA Regiment) was trapped at the base of the Chu Pong Massif. Acting on this intelligence, at eleven a.m. helicopters dropped the 450 men of the First Battalion, Seventh Cavalry, commanded by Lieutenant Colonel Harold Moore, at landing zone (LZ) X-Ray, in front of the Chu Pong Massif, to destroy the enemy force.9
But SIGINT can sometimes be wrong. As immortalized in the book and movie We Were Soldiers Once . . . and Young, Hal Moore discovered almost immediately that he was facing not an NVA battalion, but rather two full regiments of the 325th NVA Division. Two days of fierce and bloody fighting ensued, much of it hand-to-hand. When it was over, both of the NVA regiments had for all intents and purposes been destroyed, with the survivors retreating across the border into Cambodia. But the Battle of LZ X-Ray, the first engagement of the Vietnam War between American and North Vietnamese troops, showed that the North Vietnamese couldstand and fight against the better-armed Americans.
As in Operation Starlight, SIGINT’s per formance during the Battle of the Ia Drang Valley was not a complete success, with a declassified NSA history reporting, “At least four times during the struggle, South Vietnamese and American units had been ambushed by large communist units—twice during he licop ter landings—and SIGINT had been unable to detect the traps.” The lesson learned from these two battles was that SIGINT was an imperfect intelligence source if used all by itself, without supporting intelligence from agents, POWs, and captured documents. Sadly, as we shall see, this simple truth was forgotten by later generations of senior U.S. field commanders in Vietnam.10
SIGINT Successes in the Ground War in South Vietnam
While the Rolling Thunder bombing campaign in North Vietnam continued into 1966, in South Vietnam NSA was beginning to rack up some impressive gains. The list of the agency’s targets grew rapidly in response to customers’ demands for more and better intelligence, including information on the deployments and movements of North Vietnamese and Viet Cong forces down to the tactical level, North Vietnamese fighter activities and surface-to-air missile locations and readiness levels, Soviet and Chinese weapon and supply shipments to North Vietnam, North Vietnamese weather forecasts, civil aviation flights, and on and on.
And despite its inability to crack the North Vietnamese military’s high-level ciphers, NSA was increasingly able to produce vast quantities of intelligence about the North Vietnamese and to a lesser degree the Viet Cong forces operating inside South Vietnam by cracking their low-level cipher systems, as well as making use of increasingly expert traffic analysis and direction-finding data obtained by army and air force ARDF aircraft. Throughout the war, according to a declassified NSA history, “American and Allied cryptologists would be able to exploit lower level communist cryptographic systems, that is, more precisely, ciphers and codes used by operational and tactical-level units, usually regiment and below, on an almost routine basis. In fact, the volume of the so-called low-to-medium-grade systems exploited by NSA was so great that by 1968 the exploitation had to be automated.”11
This success quickly translated into better intelligence about the strength and capabilities of the enemy. A declassified May 1966 Defense Intelligence Agency (DIA) order of battle estimate of the North Vietnamese military shows that SIGINT was able to identify the locations of virtually every major North Vietnamese combat unit stationed in North and South Vietnam, as well as the locations and complete aircraft inventory for every regiment in the North Vietnamese air force.12
On the battlefield in South Vietnam, SIGINT quickly outstripped other intelligence sources in its ability to find and accurately track the movements of the ever-elusive North Vietnamese and Viet Cong forces, which made destroying them immeasurably easier. Jim Lairson, an army Morse intercept operator based at the huge Phu Bai listening post, in northern South Vietnam, recalled an incident in February 1966, when the intercepts of the Viet Cong combat unit he was assigned to monitor began moving inexorably toward his post. He remembered, “The [enemy] operator I was copying got frustrated with [his] control and switched from coded to plain text. Our translator wasstanding behind me and as I typed Phu Bai on the paper. I got the word. There were three battalions of Viet Cong coming at us.” The approaching enemy force was immediately hit by dozens of bombs dropped by an on-call force of U.S. Air Force fighter-bombers, and the threat to the base passed.13
One of the most skilled users of SIGINT in Vietnam was Major General William DePuy. Commander of the First Infantry Division, based north of Saigon, he owed his skills largely to his experience in the intelligence field before coming to Vietnam. In July 1966, army ARDF aircraft located the headquarters of the 272nd Regiment of the Ninth Viet Cong Division near the village of Minh Thanh, in Tay Ninh Province near the Cambodian border. In the resulting battle, troops belonging to DePuy’s division surprised the Viet Cong regiment, killing three hundred VC soldiers and putting the entire Ninth VC Division out of action for the next three and a half months.14
Three weeks later, in August, U.S. Air Force EC-47 ARDF aircraft flying over Quang Tri Province, in the northernmost part of South Vietnam, intercepted the largest number of NVA transm
itter fixes found in the DMZ since America’s entry into the war. The radio emitters belonged to the North Vietnamese 324B Division, which was in the process of trying to flee back across the DMZ into North Vietnam after being mauled by U.S. Marine Corps units earlier that month. B-52 bombers were called in to plaster the locations of the 324B Division with carpet bombing. Hundreds of NVA troops died in the resulting conflagration of high-explosive ordnance and napalm. The director of intelligence of U.S. Pacific Command reported on September 29, “Without [EC-47’s] work and that of more sensitive intelligence [SIGINT], we would be completely in the dark about the enemy situation in the DMZ.”15
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.20
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.21
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].”22
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
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