That and the whole Rape of Nanking thing...think that's still a sore spot between Japan and China.
It really comes down to which China values more: a "buffer" between democratic territories and itself, or the incredibly lucrative business deals it has with the West. An open door allows traffic both ways, and they depend on us as much as we depend on them, that's what cooperation's all about. Honestly, if North Korea starts serious shit, I could actually see China going in there itself and busting heads since North Korea's kind of in its own back yard, and if a kid starts trying to set the neighbor's house on fire, you're gonna want to go over there and beat his ass before he does any more damage.