Oil.
China and America hold last rights to oil found when it is running on the lower third-reserve. Once that gets really low, they will use separate military reserves to occupy, obtain and transport that oil back to the respective countries in order to maintain their internal-workings.
Chinese and American forces will fight for land laden with oil, oil-rigs, oil-derrick fields and any storage areas. The Middle-East, especially the UAE will use conventional and nuclear arms to prevent incursion onto their oil fields; thus contaminating them in the process, most likely.
They wont attack America or China directly, but rather use low-megaton yield warheads to attack formations.
NATO will back America, Russia will probably join in itself or ally with China, and China itself will be able to hold their own. India and Pakistan will probably fall into a war and **** will break down.
But yes, the Third World War will be fought over oil.