Analysis of information sources in references of the Wikipedia article "Terminal High Altitude Area Defense" in English language version.
...Japan's reversal on the Aegis Ashore sites may indicate a larger shift in defense priorities for the country and potentially signal a transitional trend with implications beyond Japan and the Indo-Pacific region.
THAAD is inter-operable with other BMDS, making it possible to have an integrated air and missile defense system. THAAD can also be used against weapons of mass destruction, i.e., chemical, nuclear and biological warheads.
...according to the Japanese government, the primary reason for not proceeding with deployment at this time pertains to environmental concerns: specifically, the matter of spent boosters from Standard Missile 3 interceptor missiles potentially dropping over inhabited territory.
...the Japanese government has been dealing with fierce local opposition to the deployment of the sites. The candidate sites in Akita and Yamaguchi prefectures weren't too popular with locals, who had concerns that the sites would all but ensure that their homes would be high-value targets for North Korean missiles in a conflict.
It's possible that Tokyo may revisit the option to procure THAAD, but cost was one factor in the Aegis Ashore suspension too. Given that the sticker price for the six THAAD batteries was greater than that for two Aegis Ashore sites back in the 2017 evaluations, it's unlikely that Tokyo will go this way.
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