Limitation of Japanese military forces was Kiku’s idea. Not Japan, but the person within.
After seeing what he had done to his friends as well as his only family, he didn’t want the temptation ever again.
Okay so we all know in reality this isn’t true but that’s what a headcanon is, isn’t it? Anyways I heard that (even now) Japanese military was limited ever since WWII so that they could never have that much military might ever again.