U.S. military planners have discovered that China's current arsenal of non-nuclear ballistic and cruise missiles could probably knock out five of six major American air bases in Japanese and South Korea. To make matters worse, this has become an issue as North Korea stumbles towards political collapse, and China indicates that it will assume control in the north if that happens. South Korea believes it should move north to deal with a collapse, and this plan is becoming a contentious issue with China.
China could use many of these missiles against Japanese and South Korean targets, although many of the missiles would have to be moved to new firing positions first. That would probably be noticed by the Americans, or Taiwanese. If North Korea showed signs of political collapse, and China began moving its shorter range ballistic missiles north, American commanders would have to prepare for the worst.
The U.S. has land based Patriot missile systems that fire the PAC-3 anti-missile missile, as well as the ship based Aegis system. But even with these defenses, the Chinese still have a good shot at winning a quick victory. Or at least crippling American and South Korean air power as Chinese troops occupy North Korea. (Source: James Dunnigan, Strategy Page, Nov 24, 2010).