Finally, a real intelligence failure on the NVA‘s part contributed to the US failure. The main reason US analysts were sure the North Vietnamese lacked the forces was because the NVA did, in fact, lack the forces. They called Tet the “general uprising”, and they were counting on widespread popular support — including, it seems, entire ARVN units defecting. That’s the only way they’d have sufficient force to knock ARVN out of the war …
… and it didn’t happen, because they, the North Vietnamese, had faulty intel.
The Americans suffered from the “intel to order” problem too, of course, which we in the civilian world call “telling the boss what he wants to hear”. But the NVA had it much worse, since that’s a much greater structural problem among Commies. Indeed, the Americans got at least one high-level defector during Tet — a lieutenant colonel I think — who only defected because the units he was supposed to command in the “general uprising” didn’t exist. They were purely paper fantasies, straight out of some commissar’s head.
And that’s what made [US Army military analyst Joseph] Hovey’s report so easy to dismiss. Hovey himself said it — it looks like they’re planning to do X, Y, and Z, but that would only make sense if they’re making a big mistake about the balance of forces. The US had pretty good intel on the ARVN and the political mood of South Vietnam. But they for some reason assumed that the NVA had basically the same information, so all of the NVA’s calls for a general uprising — which the NVA absolutely meant, and indeed were counting on — were easy for US analysts to dismiss as mere propaganda.
Severian, “Book Rec: Tet, Intelligence Failure”, Founding Questions, 2022-06-30.