China is a rival power in the area, and the United States has been trying to increase it's sphere of influence over an area China feels is rightfully hers. They've modeled the estimates of the strength of their army by whether if it could defeat the United States in a war. If I recall correctly, they claimed this was by 2010.
Japan, while not entirely happy over the bases in Okinawa and Aomori, is entirely reliant on the United States for military support, and is the only strong supporter of American interests in East Asia. South Korea has been trying that stupid Sunshine policy, so they've been trying to keep their distance from the United States in disputes in the region.
Judging from past American foreign policy, the United States will obviously try to curry favor with China.