I'm Japanese-American, so I can throw two cents in your hat.

Post-war Japan is against nuclear weapons to an absolute, but it must be admitted that the response to nukes in particular is just as much a kneejerk reaction. NHK literally spams the entirety of August with anti-nuclear propaganda every year. Japan's anti-nuclear stance is also hypocritically at odds with relying on the US nuclear umbrella for national security.

More rationally, post-war Japan is against wars of any and all kinds to an absolute. This goes as far as refusing to defend the US in the event of an attack on the US-Japan alliance; this was only changed recently in the last decade or so after strong pressure from the US to reciprocate the US's defense commitments to Japan.

Nationalism is a... complex topic. You will be considered a crazy person if you wave the Japanese flag or put up a flagpole on or around your house, but at the same time loyalty and reverence to the Emperor still remains strong and the country is politically and culturally very conservative/liberal with a very interesting mix of individualism and conformity. Most Japanese ex-pats actually leave Japan because they are more progressive and can't stand the conservative culture.

Japan is actually quite welcoming of foreigners, but there is a hard gentlemen's agreement that if you're in Japan you do as the Japanese do. Those who can adapt are welcomed, those who can't/don't are excluded and ejected sooner or later.