China's Monroe Doctrine moment coming?
With the death of North Korea's Dear Leader, and the succession of his youngest son ( the great successor - natch!) moving forward but yet to be assured, China holds the key to what N. Korea's future will be. Any legitimate successor to the throne must secure China's acceptance of their rule in addition to the agreement of N. Korea's military. This should be a simple matter as China is thought to have a fair bit of influence over its southern neighbor.
However, if N. Korea's powerful military balks and proposes a powersharing arrangement, things could degenerate quickly. And it won't take much to collapse the shaky edifice that N.K. has become. Should that happen, the US and S. Korea have contingency plans to rush north and stabilize things as best as possible -- and to take control of any nukes the North has. China understandably has similar plans to rush its forces south to forestall a massive refugee influx spreading chaos into China.
As N. Korea has long been China's client state -- China even fought for the North -- it's likely that China will see N. Korea as its special project and will not accept "meddling" from the US in what amounts to China's backyard. Such a conflict could well see the emergence of an official Monroe Doctrine-like statement for N. Korea, which China will later seek to expand to more of Southeast Asia. Any such declaration would be unassailable. China has a huge military whereas the US has fewer than 30,000 troops stationed in S. Korea. At the most the US and S. Korea would venture a few tens of kilometers north to set up refugee camps as well as rounding up as many WMDs as possible. Thankfully, most WMDs are thought to be close to the DMZ, so this can be accomplished without too much interference assuming China doesn't become overly prickly.
How the US responds will be telling. Certainly both sides will be discussing and coordinating responses to a sudden collapse of N. Korea. Let's hope that China is fully forthcoming about its intentions.