China and North Korea

Since America has been fighting the war on terror in Iraq, We have had our eyes off of the other problems. China has been sending troops, and other types of support to North Korea. Beefing up their army, but for what? Why does China feel the need to send troops to North Korea? What are they planning?
This has South Korea worried. An invasion of South Korea would cripple the South Korean economy. The situation is comparable to East and West Germany. except with nuclear capabilities. (granted East Germany was nuclear capable, allied with the Ruskies.) George Bush, fearing Nuclear War, tried to open up trade with North Korea. Fuel for Nukes. we give the North Koreans fuel, they give us their weapons, but what do we do with the Chinese troops? Is WW3 around the corner? We, or the UN, need to take control of the situation before North Korea, and their leader Kim Il Jung becomes the new Adolf Hitler.