From wiki about Japan SDF.

The trauma of the lost war had produced strong pacifist sentiments among the nation, that found expression in the United States-written 1947 constitution, which, under Article 9, forever renounces war as an instrument for settling international disputes and declares that Japan will never again maintain "land, sea, or air forces or other war potential".[citation needed] Later cabinets interpreted these provisions as not denying the nation the inherent right to self-defense and, with the encouragement of the United States, developed the forces step by step.

As I said, at first and for a number of years Japan was completely demitiliarized.
We are what we repeatedly do - Aristotle