Why do we need China for this if it is a "military reaction"?
Why do we need a "joint" reaction of any kind?
Why would a joint reaction have to be with China?
Because the U.S. + China is a nuclear deterrant like the world has never seen. No argument can be made to stop Korea's nuclear weapon programs that is more effective that the fact that the U.S. and China run #1 & 2 in worldwide nuclear might, and we are not to be toyed with by a tin-pot loony like Kim Jong-Ill.
The presence of China, another communist nation and dictatorship (but one with a vested interest in containing the rogue nation to it's southern border) at the negotiating table also gives the situation a kind of good-cop/bad-cop attitude that we may need if we are to convince Jong-Ill that giving up the nukes is his only option.
"Trust but Verify".