Ever since WW2, Japan has had no official military due to its constitution prohibiting this. Given they were an empire with such an aggressive policy towards military expansion, this can't have been a simple transition. Shinzo Abe has failed to amend the constitution despite his best efforts, and it seems to be mostly down to internal resistance to the idea, which suggests that the change has taken root on a cultural basis, and was not just something agreed to at gunpoint.
How did the allies enforce this constitutional change, and why did public sentiment not revert back to its imperial tendencies once the heat was off from WW2?
You may enjoy this previous answer of mine which discusses some of the factors that went into Japan's anti-militarism. I don't go into detail on why the far-right's attempts to revert during the 20th century failed, so more can definitely be said.