Of particular interest in recent days are the sea lanes China is working to find ways to protect. As you can see from the following (which just reference crude oil shipments) these lanes are heavily travelled. In the first chart, I have marked U.S. allies in blue (yes, Singapore is oversized) and areas that China is making claims or working to establish relations as red bursts. Note that the red bursts sit athwart the sea lanes. The second chart is from 2004 and you should be able to detect an increase in crude being shipped.
Chokepoints: Maritime Economic Concerns in Southeast Asia Institute for National Strategic Studies, Washington,D.C. (National Defense University, 1996)
(color and bursts added)
from Straits, Passages and Chokepoints: A Maritime Petroleum Distribution by Jean-Paul Rodrigue Rodrigue. Similar charts exist for other raw materials, such as metals.
So? Well, as Parapundit put it so well:
Oil is China's Achilles Heel from the standpoint of military strategy. Even if they use their massive economic growth rate to build a much larger blue water navy (and I expect they will do exactly that) it is far easier to deny the use of the oceans to some nation than to protect the sea lanes. On the other hand, even if the US and China clash over Taiwan the US would have a difficult time denying oil to China while still allowing oil to get through to other nations in East Asia. Though conceivably the US could allow tankers with carefully selected crews of known loyalties to go around New Guinea headed toward Japan and South Korea.
China wants to be in a position to defend its trade route/sea lanes and they are taking steps now to do so.
The fact that they are also vital to Japan, South Korea and lots of other countries is problematic not only for the reasons Parapunit points out, but because the potentially impacted countries are being targeted individually in what really is a collective problem. The U.S. is interested for economic and security reasons.
And that's why I keep posting about it.