The US were extremely willing to cut North Vietnamese supply lines, including illegal operations in neighbouring countries. The only thing not tried was flatten

The US were extremely willing to cut North Vietnamese supply lines, including illegal operations in neighbouring countries. The only thing not tried was flattening cities in the North by air (that approach didn't work in WW2 neither, so no big loss). True so, the South received billions in support from the US, including a full troop deployment for years. The North did the same from China and the USSR. Still, at the core, it was as much a conventional as it was an unconventional war. One the US lost. The Vietnamese did deploy a ton of cutting edge weapons so, from top notch fighter jets to air-to-air missiles. And no, you do not win those kind of wars by brutality. People love to believe that, it just doesn't work that way.