Japan was prohibited for having an offensive army. And even if that wasn't the case, it hasn't made much economic progress for the past 30 years and given the international mood after the end of the second World War, having an offensive military won't have done much to augment its growth. If anything, COVID-19 made it much worse. I don't believe it would dare try to invade China as it has grown much stronger since its last battle with Japan, maybe Korea but the two countries are as of now pretty matched in terms of offensive/defensive forces since Korea has the USA on its side. So, should any invade the other the consequences will be too large for either side to say "worth it" after being destroyed top to bottom anyways.