But what is there to do about North Korea? Sanctions won't make them back down, and an invasion is clearly out of the question - I don't think anybody, China, South Korea, Japan or the US wants to see a 10 million strong army being thrown around. And I don't know how easy it would be for the CIA to find "our guys" to support.
Iran seems easier to manage, as they have greater power in their region and can have more "responsibility" to their allies, the regional economy and so on to put it one way. North Korea only has official relations China, so if anything goes crazy, the other stakeholders most likely will get the news from China - or not at all within a reasonable timeframe - and international broken telephone is not fun.
Even though China has relatively the biggest say in what happens in North Korea compared to all other stakeholders, I don't think the North Koreans especially appreciate this. So if China was to step up its game, North Korea could do something stupid like another missile test, this time towards the middle of the Pacific, or blowing up another South Korean ship and so discredit China's ability to be a moderating force and scare the crap out of everybody else who has just lost faith in the one country believed most able to keep North Korea in line.
China would object to a US invasion of North Korea, and this would have horrible implications on their relations. Sure, even if China chooses to sit out, it's still choosing the less crappier of two crap choices. I think China would feel that such an invasion would force them to deal with millions of refugees, participate in a costly reconstruction, and a shaken-up power dynamic in the region that it did not ask for. And I don't think the US would want to act this way towards a key ally. An invasion by the US passes the bulk of the clean-up to China, no matter if it's in the US interest of security to remove the NK military threat.
I'm not even sure if China has a plan B in case of a US invasion. And we have no idea what such an invasion would look like. A unilateral invasion without consulting China is a huge insult to a major player that has much more to lose - and would most likely not happen. A unilateral invasion by the US means that the US will get all of the say in the aftermath and so China will join in the war too. But will both countries be willing to have separate campaigns without communicating with one another? That's just asking for friendly fire and screwups. But China and the US invading together just seems wrong... I don't think any amount of propaganda/media framing of the issue will change the impression of two world powers beating the crap out of this minor country for the sake of ensuring their own national interests will have a say - it's a bit of a stretch to claim "for the greater good" - and I'm sure all of us can agree that a resolution of the NK problem that is either pro-Chinese or pro-American is undesirable to all, especially because South Korea and Japan are very capable of being pissed at the US, to say nothing of their rivalry with China.
Although there might be a million ways to resolve this, China and South Korea would probably have to deal with most of the crap for 999,999 of them and so fortunately this will only give an incentive for all stakeholder nations to work closer than ever.