It could be as simple as the US withdrawing troops from the Korean peninsula once reunification does happen. South Koreans are already very pro-US, and I imagin

It could be as simple as the US withdrawing troops from the Korean peninsula once reunification does happen. South Koreans are already very pro-US, and I imagine China really doesn't want to cause an international incident by trying to annex Korea. I might be wrong however. China is very greedy when it comes to territories it has historically regarded as tributaries, and Korea has been one for thousands of years. Maybe they would use that excuse for an outright invasion, like they did in Tibet?