According to press reports, Japan intends to mount an "independent" missile defense so as not to violate a constitutional interpretation that prohibits Japan from engaging in collective self-defense. Thus Japan would refrain from shooting down missiles that pass over Japan but are targeted at "other countries," meaning the United States. Japan's political and bureaucratic leaders seem to think that such posturing is necessary to contain political controversy over missile defense. But the notion of "independent" missile defense is pure snake oil.
For Japan, it's either cooperation with the U.S. in missile defense or nothing. America is light years ahead in missile defense technology. And Japan's economy hit a brick wall more than a decade ago. Moreover, Japan is entirely dependent on U.S. sensors and infrared satellites (which detect heat plumes when missiles are launched.) Japan's own optical satellites merely replicate capabilities that can now be purchased commercially at much lower cost. Another boondoggle on the taxpayer.
North Korea, China's quasi ally, awoke Japan from its long slumber on security issues by launching a Taepodong long-range missile over Japan in 1998. That meant that all of Japan is now subject to attack from North Korea, potentially with nuclear weapons. North Korea's ruthless behavior in kidnapping and murdering Japanese citizens is also now well known to the Japanese public. That kind of regime in control of weapons of mass destruction is a threat that no government in Tokyo can afford to ignore. And Japan cannot possibly build missile defenses on its own.