[updated to clarify paragraph on Tonkin Gulf Incident]
During the Vietnam War, North Vietnamese intelligence units sometimes succeeded in penetrating Allied communications systems, and they could monitor Allied message traffic from within. But sometimes they did more than that.
On several occasions “the communists were able, by communicating on Allied radio nets, to call in Allied artillery or air strikes on American units.”
That is just one passing observation (at p. 392) in an exhaustive history of American signals intelligence (SIGINT) in the Vietnam War that has just been declassified and released by the National Security Agency.
From the first intercepted cable — a 1945 message from Ho Chi Minh to Joseph Stalin — to the final evacuation of SIGINT personnel from Saigon, the 500-page NSA volume, called “Spartans in Darkness,” retells the history of the Vietnam War from the perspective of signals intelligence.
The most sensational part of the history (which was excerpted and disclosed by the NSA two years ago) is the recounting of the 1964 Gulf of Tonkin Incident, in which a second reported North Vietnamese attack on U.S. forces, following another attack two days before, triggered a major escalation of the war. The author demonstrates that not only is it not true, as Secretary of Defense Robert McNamara told Congress, that the evidence of a second attack was “unimpeachable,” but that to the contrary, a review of the classified signals intelligence proves that “no attack happened that night.”
Several other important Vietnam War-era episodes are elucidated by the contribution of SIGINT, including the Tet Offensive, the attempted rescue of U.S. prisoners of war from Son Tay prison, and more.
The author, Robert J. Hanyok, writes in a lively, occasionally florid style that is accessible even to those who are not well-versed in the history of SIGINT or Vietnam.
The 2002 study was released in response to a Mandatory Declassification Review request filed by Michael Ravnitzky. About 95% of the document was declassified. (Unfortunately, several of the pages were poorly reproduced by NSA and are difficult to read. A cleaner, clearer copy will need to be obtained.)
See “Spartans in Darkness: American SIGINT and the Indochina War, 1945-1975” by Robert J. Hanyok, Center for Cryptologic History, National Security Agency, 2002.
Some background on the Tonkin Gulf Incident from the National Security Archive with links to related documents may be found here.
See also Declassified Study Puts Vietnam Events in New Light by Peter Grier, Christian Science Monitor, January 9, 2008.