I'm just saying that in this particular case, starting a war with North Korea means (at the very least) an artillery bombardment of Seoul which has a population

I'm just saying that in this particular case, starting a war with North Korea means (at the very least) an artillery bombardment of Seoul which has a population of 9M+. This is a virtually unblockable attack that would (optimistically) kill hundreds of thousands of innocent civilians. Worst case scenario, Seoul gets nuked and millions die. There are other ways to collapse a regime without that kind of collateral damage. The US is not in the best position to do it, however; China is.