[CSB]

But the Pentagon assumed local adversaries wouldn't know how to exploit it, the officials said.

Yeah, and the Viet Cong were too stupid to monitor FM radio traffic and learn of B-52 ARC LIGHT missions:

Even though the principles of OPSEC go back to the beginning of warfare, formalizing OPSEC as a US doctrine began with a 1965 study called PURPLE DRAGON, ordered by the Joint Chiefs of Staff, to determine how the North Vietnamese could get early warning of ROLLING THUNDER fighter-bomber strikes against the North, and ARC LIGHT B-52 missions against the South.

The methodology used was to consider what information the adversary would need to know in order to thwart the flights and the sources from which the adversary might collect this information. In the case of ARC LIGHT missions, for example, a routine warning message to friendly troops alerted opposing personnel that B-52's were on the way. Prior to the use of artillery in a given area, a radio broadcast was sent on friendly channels, warning of the impending use of medium or heavy artillery, or air strikes, near certain grid coordinates. Only in the case of an ARC LIGHT mission, however, was the phrase "very heavy artillery" used.