The United States guarantee the security of Japan ever since the end of the WWII. Because of the very specific pacifist strategic culture of Japan this cooperation was largely symbolic for the most time.
This changed with the North Korean development and test of the Taepodong-1 ballistic missile over northern Japan and lands to the Pacific Ocean in 1998. This test served as a wake-up call for the Japanese government, which immediately started to participate on American research and development of the ballistic missiles. Ballistic missile defense is solely defensive system targeted against the threat from North Korea. The “rogue” nation is developing long-range ballistic missiles, has a nuclear weapon capability and bad proliferation record. Swap deals included cooperation with Iran. It is widely suspected that Iranian scientists participate on North Korean tests and the other way around.
Currently, the North Korea might be capable to target parts of Hawaii and Alaska. The country is also striving for the international attention, currently focused on Iran, which makes is even more unpredictable and dangerous member of the international community.
As a response Japan deployed Forward-Based X-Band Radar, has Patriot Advanced Capability-3 ballistic missile defense system and deplyed 4 Aegis ballistic missile defense ships ( JDS Kongo, JDS Chokai, JDS Myoko, and most recently JDS Kirishima).