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Not understanding the enemy is the cardinal sin of warfare. Yet the Americans in Vietnam continually made this mistake when it came to their own communications security and the ability of the North Vietnamese, and especially the Viet Cong, to extract actionable intelligence from American transmissions and to interfere with ongoing operations. The assumption was that the enemy was too technologically unsophisticated and in any event did not have the necessary English skills to cause much of a threat. There may have been a grain of truth to that assessment in the very early days of the war, but the Communist forces in Vietnam were fast learners and agile adaptors. By the time of the 1968 Tet Offensive, they were routinely monitoring our transmissions.

The Americans did have an encryption, or a scrambling device, called X-Mode, which plugged into the vehicular-mounted AN/VRC-46 radios, but it was not available for the man-carried AN/PRC- 25 radios. Thus, while secure voice communications were possible between fixed command posts and with higher headquarters, transmissions with and between the infantry units and artillery forward observers moving in the field had to be made “in the clear.”

The primary method of making it more difficult for the enemy to listen in was to change call signs and operating frequencies on a daily basis. But that procedure was unpopular with the troops in the field. The issue was one of security versus responsiveness. According to Colonel Sid Berry, who commanded the 1st Brigade, 1st Infantry Division from June 1966 to February 1967, “It simplifies communications for units and individuals to keep the same frequencies and particularly call signs. Frequent changing of call signs confuses friends more effectively than enemies.”

As the war progressed, the enemy got better, and the recorded incidents of frequency jamming and false messages increased. In December 1969, units from the 1st Infantry Division overran an enemy position near Ben Suc. It turned out to be a Communist signals intelligence element. Along with 12 soldiers, the GIs captured several American military radios and other electronic equipment, as well as documents and manuals on intercept techniques and detailed information on the communications procedures and exploitable weaknesses of American and ARVN units in the area. Prisoners also revealed that the enemy had a growing number of highly proficient English linguists. By monitoring the various nets, the enemy easily extracted intelligence on troop movements, artillery and airstrikes, landing zones, perimeter defenses and transport of the wounded.

When MACV commander General Creighton Abrams was briefed about the capture of the Ben Suc signals unit, he said: “This work is really rather startling; the attention to detail, complete accuracy, and thorough professionalism is amazing. These guys are reading our mail.”

Originally published in the February 2009 issue of Vietnam Magazine. To subscribe, click here.