I wasn't aware that there was any restriction on bombing judging by what was happening at the time. this was purely a propaganda story for the gullible.
However, all out-bombing was clearly NOT going to succeed any more than LBJ's
Escalation policy of controlled acts of violence, threats and intimidation and the victim would turn aside and give you want you want.
This doesn't succeed as populations don't react like that.
I recall every escalation caused a huge increase in protests and demonstrations , with 80,000 US students considering themselves revolutionaries by 1968 and 200,000 by 1970. So an explosion would have occurred with paris type revolts in US. cities and US embassies being stormed in European capitals.
It can be imagined how the US. government could have handled this.
In fact between 1968 & mid 1972 the entireity of North Vietnam was "off limits" to U.S. bomber crews. Even when limited bombing was renewed
there were actually severe restriction on targets, some of which would have utterly devestated North Vietnam. They range from the Red River dike complex to the Haiphong Harbor and dock area to Hanoi itself. In December of '72 the U.S. took off the gloves for less than 2 weeks during
Linebacker II and hit most of the heretofore restricted target areas. The results were dramatic.
At first the NVA had noteworthy success against the B-52 strikes, North Vietnam having the heaviest air defenses in the world (at the time). As the bomber offensive continued the U.S. continually changed tactics while the Vietnamese forces fired off virtually all their SAM inventory (which, thanks to the mining of Haiphong Harbor and the destruction of the Thanh Hoa bridge in the Spring of '72 were exceeding difficult for the NVA to replace). By the eighth day of the offensive, the North was almost defenseless, thanks to alterations in American tactics and the defeat of the NVA ADZ SAM systems.
From that point forward the attacks took on the nature of a boxing match without a Standing 8 count provision. By the 28th, when the plug was pulled something like 80% of the North's electrical generation was gone, along with around 85% of the country's manufacturing base. All that the U.S. left in place & undamaged was the Red River dike system. Any sustained attack on the dikes would have flooded around 3/4 of the agricultural land in the North. Even in the case of
Linebacker II the dike system was judged to be too politically charged of a target.
There is little doubt that the U.S. could, at any time, have succeeded in this sort of bomber offensive. The question always was what good it would have done? IOTL the December bombings worked as desired, by encouraging Hanoi to return to serious bargining in Paris. If it could have "won the war", especially given the nature of the conflict, is far less certain.
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BTW: The idea that there would have been "Paris type" uprising in American cities is simply silly. The anti-war movement was remarkably peaceful and would have lost virtually ALL support had it changed nature. 200,000 people might have considered themselves "revolutionaries" but the truth is that the actual number of individuals willing to do violence is more like 1% of that total.
IIRC, my friends older brothers (and probably sisters) went to peace marches hoping to get laid, not to throw Molotov Cocktails.