This all may be true but I doubt China will do this. They’d risk a nuke lobbed at Beijing and besides it is generally bad form to attack your allies. In other words, what other countries would view China as a worthwhile ally if they think China is capable of attacking them?
Reasons for China not to attack North Korea:[ul][li]N.K. may have a nuke, and certainly has missiles that can hit Beijing.[]South Korea will complain bitterly about Chinese interference.[]United States does not want the Chinese in N.K.[]Russia does not want China in N.K.[]China can apply extreme economic pressure to achieve whatever diplomatic goals it has.China’s diplomatic clout will be greatly increased if it can broker a peaceful settlement between N.K. and U.S. (Not to mention that the U.S. will “owe” China one for the help.)[/ul][/li]U.S. has a definite national interest in maintaining the S.K. democracy. There is a vocal group against the U.S. “provoking” N.K., but I think even most of the protesters don’t want to see a sudden unilateral withdraw of U.S. troops. They mostly want S.K. to have more of say in how negotiations with N.K. proceed. They also want to renegociate the terms by which U.S. troops are deployed (particularly military vs civilian law enforcement). It would be foolish for the U.S. to withdraw in a huff because of perceived indignities.
South Corea could probably defend themselves, but with the US guaranteeing their security, it is not an issue. The North won’t attack with us there, they might if we aren’t.
A war in Corea would be bad for the US even if we aren’t involved in the shooting.