Essentially Annihilated

Tactical Nuclear Weapons in Southeast Asia : A recently declassified 1966 study performed for the Defense Department which evaluated, and rejected, the hypothetical use of tactical nuclear weapons in Vietnam. The linked page contains a copy of the actual report, as well as related analyses concerning present day implications (for terrorism, among other things). Some choice quotes:
"The use of TNW [tactical nuclear weapons] in Southeast Asia would be highly damaging to the U.S. whether or not the use remains unilateral." (7)

"The overall result of our study is to confirm the generally held opinion that the use of TNW in Southeast Asia would offer the U.S. no decisive military advantage if the use remained unilateral, and it would have strongly adverse effects if the enemy were able to use TNW in reply." (7)

"Insurgent groups everywhere in the world would take note and would try by all available means to acquire TNWs for themselves." (46)

"In sum, the political effects of U.S. first use of TNW in Vietnam would be uniformly bad and could be catastrophic." (51)
The implications of this report are arguable, but most of the analysis appears to lean towards the idea that the report is just as valid today as it was in 1966. In any case, its certainly not a bad idea to regularly revisit the issue, as the Bush administration has apparently done (against great criticism by those who don't understand or don't want to admit that military planning for unpleasant (to put it lightly) scenarios is necessary and does not constitute actual military action).