my two cents on this...
if japan seriouslu goes nuclear, us will probably leave. and no, it's not a case of japan "kicking out" americans. it would be the case of americans saying, "well,mgood luck with that! your problem is no longer my problem."
america has a nuclear umbrella over japan and south korea as many other nations. these countries in turn allow/beg/threaten to keep american soldiers there. why? because if americans get nuked, suredly america will shoot back nukes at aggressors.
but suppose japan or south korea have nukes. and let's say for arguments sake, "country A" nukes Japan. Japan would retaliate of course, but what would America do? America like everybody else would be like, "holy sh17, time to duck and cover! good luck jaoan! see ya!". then the japanese will be like, "hey you! you are my ally! you are supposed to be shooting nukes with me at country A!". and the .american response would be, "hey you have your own nukes now. dont involve us. i dont want new york city to blow up in a fiery inferno. bye bue and stop calling me."
and that is why we have the current security set up we have today. america is terrified of living in a wolprld with everyone having nukes and japan and south korea dont want to rock the boat. Now China also like this arrangement, brcausing having nukes is prestigious so they like being an "elite.". but it's also easier to deal with one nuclear country rather than a whole bunch. say china doesnt like what is happening in south korea. then they send out some dude to washington DC to be like, "yo whats up with dis sh17?!". now let's say japan and south korea also have nukes. now they have to send ipout three guys to Tokyo, Seoul, and DC and be like, "yo whatsmup with dis sh17? and btw, did you talk to the japanese about this? the koreans?". then these three guys have to call each other and compare notes and it gets really complicated and stuff.
After WWII US didnt execute Hirohito, not because american thought he was this really nice guy who was a victim of history. they let him live, because it is far easier to deal with one @ssh0le rather than one hundred million @ssh0les. BTW, this is also the problem America have with countries like Iraq and Afghanistan. America wants to deal with one @ssh0le, but there's a whole bunch of them and they dont know who to talk to, because the @ssh0le with opinions that matters keep changing.
anyway, japan goes nuclear, there will be a chain reaction of this sh17. S Korea will not sit idly and be nukeless. and then what will China do? give nukes to North Korea? they already have it. and then what happens to Taiwan? Taiwan isnt going to sit and be nukeless when south korea has it too. Then comes Vietnam, Thailand, Malaysia, Singapore, etc etc.
after thar whenenver some sh17 happens somewhere far away, we will all be sh177ing in our pants, because everyone having nukescomplicate things. it only takes one retard to suck in everybody else into some global nuclear showdown.
but whether this happens or not depends largely on what is happening with North Korea.m Can US, sKorea, japan, china, and russia all sit down and come up with a solution? things look bad right now.