Mister Kim as far as we know is kinda pretty irritable grown up kid. What his father would have allowed is sure to become the grain of sand. The 'limited strike' debated by the US in the hope of only teaching a lesson is the best recipe to get an all out strike as an answer, including some nuclear weapons.
But what if the US, considering that a strike would barely reach the West coast if accurate, has decided that an attack on the country's soil would be the best remedy to all the problems the system is in or will get into ? Answering and winning such a war would be a better symbol than 9/11 wouldn't it ?
But here comes China that doesn't answer to the same imperatives and that would well like to extand Beijing a little to the east, just what Russia did with Crimea. Moreover, at this stage where the US have lost the Syrian war, China will not allow a foreign intervention in its backyard of the loser when China sided with the winners. If there is to be a war here, it is China's business, no one else ! The case is closed.
Also, China doesn't want a Korean unification that would drag NK out of its sphere of influence and give huge economical power to a new Korea.
Obviously, Europe would support a Chinese invasion but condemn an American aggression. A Chinese victory would be the second one in a row for the new leading mafia of the world succeeding 3 centuries of Anglo-Saxons ... not ready to quit that is. But China has been an empire and never ceased to be one in the Great People Republic's minds !!! All in all, the big, the bigger, the biggest red button might be Xi's.