War is an extension of politics, by other means. In layman's terms, its just another chapter of the same book, not an entirely new book. The same political considerations matter.If you're going to drag your country to war, than military considerations rule. Otherwise conduct "war" by other means. If Nixon's air war was so effective why wouldn't a combined all out naval, air and ground assault on Hanoi break the back of the North? Political risk comes from a war that drags on and on with no obvious progress. Yes that's the war Johnson waged. I'm just saying it's not the war he should have waged
For the Vietnam War, a combined air, land, naval attack on the North might have triggered a Chinese response. China had nukes. Russia had nukes. It could literally have triggered WW3, with all the horribleness of what a global thermonuclear war would present. How can we be sure it would? We can't. So it comes down to risk.
Limited war is about limited risk. LBJ was clearly willing and able to fight a limited war without risking escalating it. But he wasn't willing to risk much. He could have done more. How do we know? Because Nixon stepped it up and did far more than LBJ, and while PRC and USSR weren't happy wiht the US escalating the war they never had the provocation to intervene. Besides that, it was grossly apparent just by watching the American evening news that regardless of whatever Nixon did, America's days of supporting the South were very numbered.
The Korean War was the same way. Had we done more, it might have triggered WW3, at a time when the USSR had nukes and the force composition to rather easily overrun Europe. Would our nukes have won that version of WW3? Probably, but it would have been very costly. So Truman, Bradley, Marshall, then Ike, none wanted to deal with that threat, so they kept it limited.