Say that, in the weeks leading up to Tet, an NVA officer is captured, interrogated, and reveals the plans to launch a massive offensive on the Tet New Year. The U.S. and the South Vietnamese promptly organise to be ready for the assault.
How does this affect the Tet Offensive, and the Vietnam War as a whole? What sort of reprecussions are there in terms of the Home Front in the U.S.?
By prepared, you mean SEATO broke up or flipped some or all of the VC cells operating in the cities, eliminating most of the Saigon carnage, etc... plus having ambush forces in place to trap and destroy/force a surrender of NVA forces attacking Khe Sahn and Hue, etc..., turning it into a full blown catastrophe, both operationally AND in the public perception?
If the SEATO forces just mop the floor with both the NVA and VC, U.S. public opinion would actually be strengthened. Rather than an obscured view of a victory, they'd be watching an ass kicking of the North live on national TV. With a clear end in sight, a major push to land a "knock out blow" might gain momentum in the U.S. and the north would be in rough shape to take it.
Won't save Johnson though.
Johnson simply would not do what was necessary to exploit such a victory: Linebacker style air campaigns to force the north to the table for a concrete peace.
He'd just stick with Rolling Thunder and if he runs, he loses when the north, even with a crippling military defeat in the south to swallow would still have absolutely no motivation to negotiate terms for a lasting peace.
Opens the door for a GOP challenger willing to go with a Linebacker/Linebacker 2 style campaign against the north to bring the conflict to an end.
Probably still get Nixon in '68. Linebacker in '69. Linbacker 2 in '70 if the north still won't negotiate and a formal peace between north and south Vietnam by '71 or '72.
There's way too many butterflies in the political realm to address here though if Tet gets squashed badly though. LBJ running for another term alone probably keeps RFK from running until '72 (thus butterflying his assassination, probably) at the earliest, with '76 being more likely.
That's just one possibility of many.