China has most to lose here (in the long run) and they have the most influence over NK by way of there economic aid to NK.

It continues to amaze me how badly China has misplayed its hand here. "IF" NK "tests" Japan will go nuclear, given Chinese & Japanese history China can't be happy about that.

Someday Korea will be unified, when that happens the unified Korea is going to look a lot more like SK then NK. With NK bringing nuclear weapons to the "unified Korea" China is going to have 2 pro-western nuclear countries on its door step. Then the pressure will build to give the Bomb to Taiwan which will put China and the USA in a very, very bad position.

One good thing is if China was to ever send "aid" to a "unified Korea", it wouldn't have to worry about a "unified Korea" stealing the locomotives and not returning them, like NK does.