The news that South Vietnamese officials flashed from I Corps last week was nothing less than astonishing: swarms of North Vietnamese helicopters had been sighted in the Demilitarized Zone, they claimed, and more than a dozen had been brought down by allied fire. Thus began the Great Helicopter Mystery.
Beginning two weeks ago and lasting for several nights, allied counter-mortar radar along the eastern edge of the DMZ, where the zone is bordered by the South China Sea, had indeed showed blips that looked like slow-moving, low-flying aircraft—like helicopters. American artillerymen had also reported sighting a series of strange moving lights near...