I never really understood - apparently only someone as smart as McNamara could.
So far as I got it the reason was if we really went to war against the North (as they had gone to war against us in the South) we might make them really mad at us, or might bring in the Chicoms or might bring criticism from the "international community." Plus, McNamara seemed to think if he only applied a little pressure and then gradually increased pressure against the North they'd come to their senses and negotiate a peace deal.
Then there were various suspensions of bombing what targets McNamara would allow us to bomb to give the North an opportunity to sue for peace, none of which went anywhere.
If Obama could ever bring himself to go to war, his ilk would probably fight it in a similar fashion, with one hand tied behind his back.
“So far as I got it the reason was if we really went to war against the North (as they had gone to war against us in the South) we might make them really mad at us, or might bring in the Chicoms or might bring criticism from the “international community.” Plus, McNamara seemed to think if he only applied a little pressure and then gradually increased pressure against the North they’d come to their senses and negotiate a peace deal.”
I think your second point is more important. McNamara personally chose targets, ordnance, and even approach and departure routes as a way of sending various “signals” to the NVN. For instance, when the first SAM sites went up in NVN in the spring of 1965, McNamara in his folly placed them off-limits as a way of “signaling” the enemy not to use the missiles. The NVN, of course, were actually under attack and such an idea probably never occurred to them, except to shake their heads in wonderment at the stupidity of the American leadership. Only after several US aircraft were shot down did McNamara respond and order the sites attacked, giving plenty of opportunity for them to be moved or reinforced into “flak traps” in the meantime. Losses were heavy in the resulting strikes.
The magnitude of the attacks was more than adequate to do the job, it was the nature of the attacks that made them ineffective.