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.

Declassified study puts Vietnam events in new light | csmonitor.com