China will not allow North Korea to start a war. If Kim Jong Ill gets too out of line, China will either take him out and replace him with somebody stable or invade North Korea themselves first and put in another puppet under the guise of doing it for the UN.
It serves China no purpose what-so-ever for Korea to be united.
1) China knows if they go to war against the UN and NATO, their economy will fold since it is being artificially held up by the dollar. They would lose the majority of their sales. All of their bonds they own from the US would become worthless because chances are the US economy would plunge further down.
2) If China stays out, North Korea will lose and Korea would be united under a pro-western government and have a combined military larger then China's.
3) China already has a problem with refugees fleeing from North Korea to China, a war would mean a lot more North Koreans flee to China then already are.
By taking out Kim or invading North Korea first, they retain control over the rebuilding and keep their safety buffer on one of the main historical paths of invasion into China.
If South Korea attacks first (which is no interest at all to South Korea, especially with the capital is in artillery range of North Korea), then China could come to the aid of their "ally" against an aggressor and control the rebuilding of North Korea after the war.
So don't worry, it wont be World War III.