PM Abe (who was returned to power with a reduced mandate) has just managed to convince his coalition to at least consider a Constitutional change allowing Japan to participate in regional defense activities beyond their borders. A war declaration without an attack is simply not within the realm of possibility despite what many warhawks here seem to want.
At this point. Let Japan have a full military. They already have so many potential threats in their own backyard, I doubt that they could achieve the level of empire that they had in World War II.
I don’t think Japan would need to go to war, just debating it would likely force China to do some serious soul searching in regards to North Korea.
Japanese soldiers created a lot of lasting hatred in both Korea’s and China. Nobody over there wants the Japanese declaring war on North Korea and yet Japan’s interests are being threatened by North Korea.