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. Oops. 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.
It's long been obvious that China planned a similar tactic against Taiwan. Wargames and detailed analysis of possible Chinese attacks on Taiwan, indicated that the basic Chinese missile attack strategy might work, and do so within days. The key to such a blitz is the 1,300 Chinese ballistic and cruise missiles. Most of these are based on the coast opposite Taiwan (180 kilometers away across the Taiwan Straits). The Chinese missiles carry one ton or half ton conventional (high explosive or cluster bomb) warheads, and were expected to be used to try and cripple Taiwanese air force and navy, as well as attacking headquarters and communications targets. Almost simultaneously, China would try to invade with airborne and amphibious forces.
Without those missiles, Taiwans's superior air and naval forces would make it very difficult, if not impossible, for the invasion force to cross the straits. The wargames play out various targeting strategies, and defensive moves the Taiwanese could take. In most cases, the Chinese succeed. The barrage of missiles do serious damage to Taiwanese air and naval forces, giving Chinese air and naval forces an opportunity to get ground forces ashore.
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.