The demilitarization of Post war Japan was Important because it would Strictly control Japan's ability to Repeat an event such as Pearl Harbor. it also enabled America to trust japan more without fear of Retaliation for the Atomic Strikes of Nagasaki and Hiroshima, And to Keep them In Check as you might say. Japan cannot Have a standing Military, However they may have a Defense force (JSDF: Japanese Self Defense Force) to keep their Mainland safe from Enemy Attack.
