Declassified study puts Vietnam events in new light

US signals intelligence – the much-vaunted ability of American military and spy units to eavesdrop on the radio calls and other electronic communications of an adversary – failed at crucial moments during the Vietnam War, according to a just-declassified National Security Agency history of the effort.

The 10,000 cryptographers and other signals personnel in Southeast Asia at the time did not predict the start of the Tet offensive on Jan. 31, 1968. Prior to that, signals intelligence may have actually misled President Johnson and other top policymakers about the nature of the 1964 Gulf of Tonkin incident, in which a supposed North Vietnamese attack on US forces triggered a major escalation in the war.

US eavesdroppers had many successes during the war, according to the lengthy document, particularly in picking up the tactical communications of North Vietnamese and Viet Cong fighters in the field.

But when it comes to major events, signals intelligence is not magic, as the history makes clear. That is a point current policymakers would do well to remember as they struggle to interpret intelligence dealing with the complex modern problems of nuclear proliferation and Islamist extremism.

In both the Tet and Gulf of Tonkin cases, "critical information was mishandled, misinterpreted, lost, or ignored," writes NSA historian Robert Hanyok in the agency history.

Yet both were major turning points of the Vietnam conflict. The Gulf of Tonkin led to open US involvement in the fighting. Tet, though a tactical military defeat for the North, was a surprise for a US public that had been led to believe victory might be imminent. It may have contributed to declining support for the American intervention.

The Gulf of Tonkin incident occurred in early August 1964. On Aug. 2, North Vietnamese patrol boats attacked a US destroyer, the USS Maddox, in the Gulf of Tonkin, an arm of the South China Sea off Vietnam's northeastern coast. Mr. Johnson warned the North that another such attack would bring "grave consequences." On Aug. 4, Johnson announced that another attack had occurred and asked Congress to vote him powers to respond. On Aug. 7, Congress gave him those powers in the Gulf of Tonkin resolution, which became the legal foundation for increased US involvement.

Even at the time, some doubted that the second attack had occurred. Yet the Johnson administration produced what seemed a key piece of evidence – a North Vietnamese Navy after-action report, intercepted by the NSA, which appeared to discuss the battle.

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