BEIJING  For the last several years, one big theme has dominated talk of the future of Asia: As China rises, its neighbors are being inevitably drawn into its orbit, currying favor with the region’s new hegemonic power.

The presumed loser, of course, is the United States, whose wealth and influence are being spent on the wars in Iraq and Afghanistan and whose economic troubles have eroded its standing in a more dynamic Asia.

But rising frictions between China and its neighbors in recent weeks over security issues have handed the United States an opportunity to reassert itself  one the Obama administration has been keen to take advantage of.

Washington is leaping into the middle of heated territorial disputes between China and Southeast Asian nations despite stern Chinese warnings that it mind its own business. The United States is carrying out naval exercises with South Korea in order to help Seoul rebuff threats from North Korea even though China is denouncing those exercises, saying that they intrude on areas where the Chinese military operates.