In the fall of 1969 in Bing Duong province of Vietnam, a North Vietnamese soldier sat at a table in a large underground tunnel, taking notes.
"Do you know about the heavy artillery warning yet?" he scribbled in English as an American soldier's voice crackled over his radio.
"Negative," another American responded.
"The coordinates are 550 600, 3/5/31 until 1130 hours," the first soldier said.
In the margin, the North Vietnamese soldier wrote "heavy artillery = B52 Strike, at [11:30 a.m.]" When the B-52 airplane dropped its 750-pound bombs on a target in a nearby province 20 minutes later, the enemy was long gone.
The notebook is one of hundreds of documents recently declassified by the National Security Agency depicting numerous similar situations of foiled bombing raids and ambushes during the Vietnam War that offer new insight into an underestimated enemy.
By revealing the North Vietnamese's intensive and successful efforts to breach American communications, the documents, many now on display at the NSA museum, begin to dispel the long-held assumption that the Viet Cong were technologically unsophisticated, many experts and veterans say.
For some, the disclosure is the last missing piece that helps explain how a technologically advanced nation like the United States couldn't win a war halfway around the world in a small country's jungle."[These Viet Cong] efforts impacted the war in terms of casualties and facilitating ambushes that didn't need to occur, and it allowed them to avoid contact with American forces," said Michael Jacobs, the agency's director of information assurance. "There were a number of ambushes that occurred in those days that always seemed a bit suspicious.
"When you couple [Washington's reticence about the war] with the failure to secure communications, you begin to see ... increasing casualties and great frustration over blown operations, and over time, they became reasons for the failure in Vietnam."
The documents show that beginning with the buildup of American forces in Vietnam in 1961 through the early 1970s, American commanders had little idea the North Vietnamese were exploiting their communications.
The first hint of enemy eavesdropping was uncovered when American soldiers spotted a strange antenna sticking out of the ground in 1969 and stumbled upon an intricate tunnel system housing two dozen men and "very high quality" transmitters and receivers. The soldiers found more than 1,400 recent hand-copied voice transmissions and even biographical data of hundreds of soldiers: their birthplaces, speech patterns and habits.
In a declassified 1970 report on the incident, American officials warned their commanders that their soldiers weren't properly encrypting their conversations - and in many instances weren't encrypting them at all - and that hundreds of missions had been compromised. By the time communication security improved in the early 1970s, the American war effort was already on the decline.
In one instruction manual taken from the Viet Cong and held by NSA, the North Vietnamese author wrote: "[American] exchanges are a little strange ... but they are not very fast. Officers ... when in communication with subordinate officers or units report calmly and carefully even during an attack. ... We can determine where the enemy has discovered us, whether our plans have been compromised.
"The messages they send are easy to understand, because they do not encrypt coordinates which is very advantageous for us."
John D. Bergen, who studied many of these classified reports as a lieutenant colonel in the Army and later wrote the book "The Electronic Battlefield" for the Center for Military History, said that American soldiers' encryption efforts were hampered by complicated coding schemes as well as their ubiquitous FM voice radios, which were cheap and simple to operate.
These radios had tremendous range and transmitted in every direction, much to the advantage of the Viet Cong.
American soldiers often made their own homemade codes, assuming the Vietnamese wouldn't understand baseball analogies or "Boston" to signify "east."
But unlike U.S. soldiers who rarely spent more than six months in one place, Bergen said, the Viet Cong spent years in the tunnels of numerous intercept sites, working diligently with often simple radio devices. Despite such simplicity, their notes reflected extraordinary organization, detail and sophisticated mathematics. Each Vietnamese soldier would study one American unit seven days a week for months at time, as Americans "chattered incessantly."
"We thought we were invulnerable," said Bergen, who served as a battalion adviser in Vietnam from 1968 to 1969.
"I think there is a lesson there as we engage in peace-keeping situations all around the world now," he said, "that even when you are facing a seemingly poorly armed and primitively organized entity, the worst thing you can do is underestimate the enemy, just like we did then."
For many American officials, it was difficult to believe the Viet Cong had the information they did, largely because the enemy went to great lengths to portray their decryption and technological abilities as nonexistent.
Douglas Pike, who was a Foreign Service officer at the U.S. Embassy in Saigon, remembers there being much discussion about why so many Air Force missions and bombing strikes were unsuccessful. The common thinking, he said, was that Russian trawlers were watching the airplanes take off, perhaps "guessing" where they were headed.
"That's what we said publicly," said Pike, now the director of research at the Vietnam Center at Texas Tech. "Later at the Embassy, in private briefings we wondered if there was some leakage. Some people speculated that they must have people on the ground who were listening in, but you never knew what the validity of those allegations were.
"It's a kind of arrogance really," he said. "We just couldn't believe people wearing black pajamas could mount that kind of surveillance."
But they did. Other documents confiscated late in the war show, for example, that at a surveillance site outside the 1st Calvary Division's headquarters, the Viet Cong monitored the 11th Aviation Group's helicopter unit every night, learning where the battles were to be fought the next day and setting up ambushes.
Joe P. Dunn, professor and chair of the History and Politics Department at Converse College in North Carolina, who is a national authority on how the Vietnam War is taught, said these documents will begin to change people's understanding of the war.
"It deepens our understanding of how things happened and ups the level of what they were doing that we didn't appreciate," he said. "There's a lot more to learn over the next decade about the technology and capabilities of the Viet Cong that will likely lead to a reinterpretation of the war."
Americans were also spying on the Viet Cong, though their efforts were often hampered by the Viet Cong's diligent use of encryption. The Vietnam War was also the first time the agency used airplanes - albeit cramped, small ones - to spy and gather electronic intelligence, which the cryptologists dubbed "TWA" for "Teeney Weeney Airlines. The first solider to die during the Vietnam War, according to the NSA was cryptologist James Davis, who died in an ambush outside Saigon in December 1961.
Cole Miller, a retired NSA analyst who was one of the first two cryptologists permanently assigned to Vietnam, remembers seeing Davis' wedding ring and personal items on a table when he arrived. "I just remember thinking, this is for real, folks, we're not just practicing anymore," Miller said.
Like the Viet Cong, NSA attempted to hide its presence.
Jack Barrett, also a retired analyst who served in Vietnam, said they wore Army fatigues but their badges said "Department of Defense Representative."
"Half the people would salute us and the other half didn't know what to do," he said.
The last wartime message the Viet Cong intercepted was the last message an American sent.
From the Embassy in Saigon in 1975, a cryptologist transmitted, "I have just received word to evacuate. Will cease transmission immediately after this message. We're tired, but otherwise all right. Looks like the battle for Saigon is on for real. I commend to you my people who deserve the best NSA can give them for what they have been through, but especially for what they have achieved."