If the US was to engage in such military operations, the PRC would intervene on North Korea's behalf in order to maintain a buffer state on the Korean Peninsula. In which case there would be war between the US and the PRC. Which would be quite catastrophic.
To be honest, I wonder how quick China would be to jump in on the side of North Korea once again.
Remember, they did so once before, and basically had their keisters handed to them because of it. And at that time, their economy was not dependent on export like it is today. If North Korea attacked South Korea again, it would once again not be a US operation (although the US would likely provide most of the firepower), but a UN operation. I would expect it would follow along these lines:
1. The US quickly mobilizes it's forces and responds to the conflict.
2. The UN starts having emergency meetings, issues a condemnation.
3. NATO mobilizes and responds to the conflict.
4. The UN has more meetings, issues an ultimatum.
5. Australia and other nations will respond to the conflict.
6. The UN finally gets involved, and more forces come to the aid of South Korea.
And considering the buying power of the US (not to mention the NATO nations and others that would likely come to help), the PRC will likely sit it out other then providing some covert aid. This is no longer 1950, and do not expect China to send 1.3 million troops into the grinder. And if they were foolish enough to try, expect nation after nation to place an internal embargo on China and see most of their trade come to a crashing halt.
China knows this, and they are not stupid. If they do assist N. Korea, it would be along the lines that the Soviets did 60 years ago. Some equipment, some advisors, maybe some pilots, but that is about all. They simply have to much to loose this time.
I disagree. China interfering could risk a cancellation of debt, which would end the state of China almost immediately.
That would be nothing to the effect of the loss almost immediately of over 1/3 of their import-export market. And since there are many other nations that would likely jump in as well (NATO for one), China would soon be almost completely crippled economically.
Most do not realize the house of cards that China has built itself. The majority of their economy is based upon taking or importing raw materials, and exporting finished products. The US alone is almost 1/3 of that figure, and Europe is another 1/3. So with the US and a big chunk of NATO involved in a conflict with China, do you think they will continue to trade with China?
There is an old saying, "If you owe the bank $100 that's your problem. If you owe the bank $100 million, that's the bank's problem."
In 1950, China could alienate most of the world, because their economy was small and almost entirely internal. 60 years later this is no longer the case, their economy is almost entirely dependent on foreign trade. And none of this "trade" is of critical importance to the rest of the world. COSCO furniture, ePhones, and lots of other consumer and light-luxury goods that a nation can do without and other nations can build.
Compared to what they would loose in trade and asset removal and "brain drain", the loss of "debt cancellation" is nothing.