There are really two problems.
One is North Korea themselves. This is a distant but real threat, that they will act in a rouge way, in the interests of an individual or possibly out of an inflexible system the cause huge devistation. Most certainly for south korea, probably Japan, within the realm of possibility of Russia, China and the US possibily further afield. Certainly they would seem intent to gain further advantage by nuclear ransome.
The second is that the North Korean situation may be taken advantage of by the Chinese and the Russians for there own strategic objectives. Move the US away from them and their interests, threaten by proxy for their own objectives. Arguably that is already happening.
Both aren't really acceptable to the US, as both are effectively real attacts on their position, status, interests, allies etc.
North Korea could be managed if they only had conventional weapons, the threat to US allies and China and Russia was basically very, very low. South Korea was the major state that was vunerable and for a number of reasons that was less likely to happen. Ego sit and wait, it seemed plausable that the regime might collapse anyway, most soviet style states have or have restructured in such a way that the animosity died with their leader (Cuba?). NK beat the odds.
Now the time has run out. While the US (lets face it, and others inc China and Russia) and has been very active in trying to stop North Korea fielding capable nuclear weapons, (and its been 50 years, so those efforts haven't been invain). It would now appear that North Korea is on the verge of being able to field deployable nuclear weapons on long range missiles. It can extort the US directly instead of by its allies. Not just the US either.
That is what has changed, the quality of the weapons, the range they can now effectively strike. Previously NK had some nuclear capability, but it wasn't believed to be reliable or effective enough to deploy and had no reliable long legged deployment option. With the new missiles, the new subs NK has changed the situation. It's either act now, or never ever act.
China and Russia think that they can still influence NK and that if the US withdraws everything will be fine and dandy. A variety of events make that level of influence seem laughable and even if true are still not acceptable to the US (threat by proxy).
Short of NK becoming an open, democratic society, with different leadership, reuniting with SK, a revolution or NK honestly terminating their nuclear program in a very open way it with an absolute plan for leadership change would seem we are sliding into a military actions, regardless of how much China, Russia, the US, Japan, South Korea would prefer it not to come to that.
I expect that yes, sanctions on China to apply pressure. The question is who is going to support the US on that front. The US will want this to be a team effort. I don't think the sanctions will be effective, I don't think China has that kind of influence anymore.
The most probable course of action is NK will continue their program, the leadership does not change and all sides increase levels of harrisment until something happens.
I expect a series of cold war type of events, in the air and on the sea, baiting each side and escalating.
The US will aim to side line China/Russia (ie sanctions/international pressure) and then apply direct pressure to NK.
Hopefully in 6 months I am wrong and everything works out. 60:40 I would say at this stage.