I see no way this could ever backfire!

[Anomalocaris]

The North Korean military itself would be but a speed bump to our military now. China, on the other hand, would be more formidable.

North Korean conventional military forces may be nothing more than a very low speed bump for the US or South Korean military. But a zenophobic, paranoid militarized state like North Korea almost certainly has made extensive contingence preparation for the eventuality of being overrun, and would have prepared weapon caches, partisan resistence organizations, safe houses, method to continue to intimidate the civilian populations, and the other necessecities of a lengthy and bloody insurgency. As Iraq and Afganistan shows, Americans are reasonably inept at dealing with a motivated insurgency. So even if there is no direct Chinese intervention and military action against the US, any eventual American success in military operation against North Korea would probably take much longer than one might realize when one simply compare opposing forces tank for tank and plane for plane.

Furthermore, if it really becomes clear North Korea is about to be invaded and overthrown from the South, the Chinese might just preemptively invade and occupy North Korea, and confront the US with the much tougher choice of whether to actually attack the Chinese or not.