The Chinese authorities were the first to complain when ballistic missile defense (BMD) came up as a possible option for the Japanese Self-Defense Force in the early nineties in the form of the US theater missile defense.
The National Defense Program Outline in and after FY 2005 (sorry, I can't find a translation) gives the reason for its introduction as follows:
" The thinking on the response to and the institutional arrangements of the Self-Defense Forces for the new threats [including the progress of the proliferation of weapons of mass destruction and ballistic missiles, and the activities of international terrorist organizations, etc.] and the wide variety of situations [that influence peace and safety] is as follows:
a) Response to ballistic missiles: Against ballistic missile attacks, [we] shall deal effectively by establishing the necessary institutional arrangementｓ including the deployment of a ballistic missile defense system. Concerning the threat of nuclear weapons against our nation, [we] shall deal appropriately by such undertakings, together with the nuclear deterrence powers of the United States.
It is not clear what is meant by the "wide variety of situations that influence peace and safety", but it does not explicitly exclude the rapid, continuous build-up of Chinese military capacities that the 2005- Outline lays out as follows:
"China, which has a major influence on security in this region, has pushed the modernization of its nuclear and missile military powers and naval and air power, and is also attempting to expand the areas of activity on the oceans; and it is necessary to continue to pay attention to such developments."
However, if the Chinese authorities still have serious qualms as the Japanese Ballistic Missile Defense System prepares to go into operation, the media are not paying notice.
Perhaps they have been assuaged by Japanese assurances like this:
"In relation to the right of collective self-defense, the purpose of the BMD systems that the GOJ will introduce this time is to exclusively defend Japan, and it will be operated based on Japan's decision on its own initiative and will not be used for the defense of third countries. From this point of view, no issue of the right of collective self-defense comes up." (Statement by the Chief Cabinet Secretary, "On the Introduction of Ballistic Missile Defense Systems and Other Measures " on December 19, 2003)
In other words, in the case of a military showdown over Taiwan, Japan will not dispatch those AEGIS ships equipped with SM3 missiles to shoot down Chinese ballistic missiles launched against Taiwan. (Or the US.)
On the Japanese side, at the beginning, the defense and foreign policy establishment appears to have been somewhat ambivalent, presumably over the financial costs and technical feasibility of the program. There may also have been some reluctance to antagonize China, with whom Japan had been careful to maintain good relations even after the 1989 Tienanmen Incident. However, the Nodong (1993) test in the Japan Sea and, more significantly, the Tepodong (1998) shot over Japanese air space into the Pacific Ocean appears to have galvanized domestic support. On other words, it was North Korea's nuclear and ballistic missile programs that jointly tipped the scales in the BMD's favor. At a minimum, the introduction of the BMD would have taken a more tortuous road to fruition without Kim Jong Il's help.
Now, through the Six-Party Talks, North Korea seems to be seriously seeking a new equilibrium on the Korean Peninsula. I still think that by far the most likely better-case scenario for the foreseeable future is North Korea disabling its three facilities but holding onto its current nuclear arsenal and ballistic missiles system at their current levels, enough to allow China and South Korea to keep sustaining the current North Korean regime without incurring US disfavor, but well short of normalization of the US-DPRK relations. Still, there is enough movement in the Six-Party Talks to make it worth considering: What will Japan's defense posture be like without a nuclear threat from North Korea? More specifically, what will become of the Japanese BMD System once China becomes the sole concern of direct military threats to the homeland?