North Korea is communist and South Korea is Democratic. Do you know what China is afraid of? One Guess. China is afraid that North and south Korea will unite into one country. Not only will china loose a communist ally, but it will have a free, armed county on its border. NOt only might there be an armed conflagrations, but western values may start to penetrate the Chinese mainland. After all, the Chinese wouldn't let google in China because they did not want their people to be able to search the world wide web, just the China wide web.
