"'China would have to fight its way into North Korea,' Mastro told me in an interview. 'For the North Koreans, enemy No. 1 is obviously the United States, but enemy No. 2 is China. They understand they’d have to potentially fight both countries.'
Things would get really complicated, and really dangerous, once Chinese troops made their way to the nuclear facilities. The Pentagon has spent years practicing how to send US special operations forces into North Korea to seize Pyongyang’s nuclear weapons if there were signs that Kim’s government was collapsing. The problem is that Chinese troops would almost certainly be sent into North Korea at the same time, and with the same goal, as the US forces.
Mastro notes that Chinese troops would only need to advance 60 miles into North Korea to take control of all of the country’s highest-priority nuclear sites and two-thirds of its highest-priority missile sites. Given that enormous geographic advantage, Beijing’s troops would almost certainly arrive before the US ones do."The notion that Kim Jong-un would passively "absorb" a US first strike is stupid and absurd. Seoul will be devastated, and within a few days you'd be looking at US and Chinese ground forces coming into direct contact.
That isn't a "limited strike," it's World War III, and the Russians aren't going to sit on their asses and watch this play out either.