China has told South Korea that it will not allow the unification of Korea under a democratic government. North Korea will remain under Chinese "influence." If worse comes to worse, China will send in troops to set up a North Korean government that will faithfully follow orders from China. In an effort to dampen some of the anger in South Korea (the United States, Japan and so on), China would maintain North Korea as a separate entity (and not a new province of China). China wants no misunderstanding about who "owns" North Korea.
Even aside from all of China's neighbors who will worry that China will suddenly discover that they really own part of that neighbor's territory, this could cause quite the risk of war if China carries through with the threat.
I can understand why China would not want South Korea's border on the Yalu River. China went to war with us once over that. They may not have liked the experience much, but the motivation to fight are still there. China wants a buffer zone.
But so does South Korea. For decades, an effective and large North Korean army loomed over Seoul a short distance north across the DMZ. As North Korea's armed forces have rotted away, the threat of invasion has pretty much vanished. North Korea would have to nuke and gas South Korea and pray that they can manage a road march south against minimal opposition--and then recover from the nukes we'll plant on North Korean military assets and the smart bombs on everything else.
South Korea wants to reduce their army. But they can do this only if North Korea continues to be the de facto buffer zone between Seoul and an effective army in China. If China occupies North Korea, not only will China be on South Korea's border, looming over Seoul; but China will reconstitute North Korea's army into an effective force. It will no doubt be smaller than it is now, but it would be the first wave only. China would follow up with their own army if it comes to war with South Korea. It just doesn't matter that North Korea would be officially independent and not a new province of China. Remember, Belorussia and Ukraine had their own UN seats when they were part of the Soviet Union. Does anyone think they were really sovereign states then?
I just don't think South Korea wants to be in a worse situation after the collapse of North Korea than they were before the Soviet Union fell and slowly took North Korea's military power with them. If China takes over North Korea, South Korea will face the threat of invasion again--and this time by a nuclear power that could possibly nullify our nuclear umbrella. We'd nuke North Korea if they nuked South Korea or if we couldn't handle a conventional invasion since North Korea can't reach us now. Would we nuke China--or even Chinese assets in North Korea--under similar circumstances? Maybe. But it isn't nearly as certain.
So if North Korea collapses, I'd expect South Korea to want to move their buffer zone north. Not all the way to Pyongyang, certainly, let alone to the Yalu. But north. And they could do it faster than China could move south in force. If I was South Korea, I believe I'd think it worth the risk of war with China now well north of Seoul (while America is in South Korea and before China can dig in within North Korea after a few years) rather than risk war later with Seoul on the front line.
I still think the best solution to a collapsing North Korea (or even a non-collapsing North Korea if it looks like it is going to field nuclear-tipped missiles) is partition.
South Korea and China both want to keep the other away from their border. Unless China wants to risk war with America and the disruption to their economy, giving both Seoul and Peking their wish seems the best option.
It can be portrayed as an interim solution. But we can all be vague about what comes next for North Korea's territory and when it comes. Maybe the horse will learn to sing, eh?
