Why did the United States feel it was necessary for Japan to be rebuilt after World War II?

The United States saw Japan as a security state in hopes of containing the spread of Communism. Japan was already a regional (though damaged) power, and keeping Japan from falling under Communist rule was needed to keep the sphere of influence in Asia from swinging completely to Communism. Also, as a security state, the US can continue to post troops and security task forces to help defend allies in the area.

Later on, installation of radar for early detection, and missile bases for quick action/reaction set up was placed in Japan as a deterrent against Communist forces. Japan continues to be a key US ally after World War II in Asia, and will continue to be in the years to come.

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