My understanding is that China has a very different relationship with North Korea and a very different perspective on foriegn policy. China tends to emphasize North Korea's stance as a reaction to US/South Korean provocations. China also has a significant population of Koreans and significant trade with North Korea, and fears the destabilizing effects of North Korean collapse more than the destabilizing effects of North Korean actions. Finally, China's policy is that countries should not peer to closely into the domestic affairs of foreign countries.

North Korea's strategy is to deter foreign aggression against itself. As this article reveals, it has been an effective strategy, because the costs of war on the Korean peninsula are so huge despite the North's technological deficit.

It's not a question of whether either has the military power to overrun DPRK. Both could in a matter of hours a few days. A lot of it is economic.

What happens to the ~24 million DPRK citizens who are very low skilled? Whatever country takes them on is taking on a gigantic uncertainty and costs for literally millions of people. During famines, North Koreans have literally gone to the hillsides to get grasses to eat. Many are malnourished, roadways are poor outside of Pyongyang because they need travel permits to travel within country (so the roads are not used much). To make anything of the country would require huge infrastructure costs. Many of the roadways are also suspected to be rigged with explosives so if/when a land invasion occurs, US and South Korean troops are slowed down significantly.

China doesn't want to do it because the buffer between the "US sock puppet" called South Korea is nice to have. They have had trade going with North Korea for years, and recently have gotten annoyed by the rhetoric and BS spewing from their buffer zone.

With Seoul very near the border, South Korea stands to lose many billions of dollars in assets from artillery once the invasion starts.

China does not want the US building military bases a few miles south of their border. Additionally, in the event of a conflict with the US directly, the DPRK will almost certainly seize the opportunity to cause havoc on the peninsula and therefore divert US/NATO resources away from a Chinese/US conflict.

One could argue that the US wouldn't need to maintain bases on the peninsula after the fall of the DPRK, but with the situation in the South China Sea, we'd be stupid not to and even if we agreed with China that we wouldn't, they don't have a lot of reason to trust we'd hold up our end of the bargain given our history.

DPRK is a buffer state for China.

The United States lost its mind when communism established a foothold in Cuba. Russia did likewise when Ukraine began aligning towards NATO.

China would prefer to have a stable regime in DPRK - but it would also prefer to not have US military bases on the other side of the Yalu river. The last time that happened, the two countries went to war.

Also, arguably, the DPRK regime is extremely 'stable'. The same royal family has ruled it for what, seven decades, now?

It's stable right up until it is no longer stable just as much as a boat that has been afloat for 70 years can't sink.

The current leadership does not strike me as 'stable' in any way. But then again, that goes for more world leaders.

Should some of the stunts they have pulled during that time be considered when defining "stable?"

Well, unlike some other nuclear powers, they managed to restrain themselves from invading a few sovereign nations in the past few decades.

Seoul, where almost half of population of South Korea lives, is just 40 km from the border and North Korea have a long range artillery. The war is impossible unless you are good with hundreds of thousands civilian casualties.

Will US withdraw troops from a unified Korea or will troops be stationed on the Chinese border?

The Russians withdrew from Central and Eastern Europe (save for Transnistria) yet US troops are still present in Germany and now in Europe (literally invited by the Baltic states, Poland and Romania), not to mention Japan, South Korea and Guam which is basically US territory. I highly doubt that the US would withdraw from a supposedly Unified Korea.

China-DPRK relations have only begun chilling over the last decade. China still has a lot of vested interest, both financial and political, in North Korea.

> why can't China and the US join forces and militarily overrun DPRK?

All sibling comments talking about things like buffer states are missing the forest for the trees. You don't invade North Korea mostly because they have nuclear weapons.

No sane US administration (sighs) is going to risk over a million dead civilians in Seoul, Osaka or Tokyo and the resulting global recession.

For China, despite all the missiles NK lobs toward Japan, the reality is that Beijing is closer to Pyongyang than Osaka and likely has worse missile defense. A Japan with nuclear ambitions isn't high on China's priority list either.

You can hand wave their shaky missile tech away as a threat to Japan and China but that's a lot of lives to gamble on and there's a real possibility they've smuggled weapons abroad as well (more likely chemical than nuclear but still bad).

The transition itself would dramatically increase the dangerous instability which is reason enough to try to put it off. "The horse might learn to talk" and all that.

A lot also hinges on what sort of deal China could work out with SK and the US. Would US forces come all the way to China's borders? Would the US leave the peninsula entirely after the war? Maybe China could be bribed with a lease for a nice naval base on the Sea of Japan? I'd hope that there's been some discussion of all of this ahead of times given how the last Korean War went but who knows.

How about the U.S. agrees to help integrate Taiwan back into mainland China (similar to the British and Hong Kong) in exchange for China helping with the North Korea situation?

EDIT: changed 'we' to the U.S.

> One thing I have never understood: why can't China and the US join forces and militarily overrun DPRK?

I cannot think of a regime changing American military excursion in the last 60 years that resulted in the scenario that America wanted, with the possible exception of Grenada.

That's a good reason to not invade.

China certainly doesn't want a unified Korea, for economic and military reasons.

After reading the article it seems that DPRK is a relatively tamed threat. If we invade, they will undoubtedly do something extreme, where as right now they just /might/ do something extreme.

The problem is, if you topple the Kim regime, a unification with South Korea is imminent and bringing North Korea up to the development level of South Korea is would cost trillions of dollars. It might even trigger a worldwide economic recession as the capital markets completely dry up as all investment floods to Korea.