I think Japan might be secretly working with the NK government to build their own nuclear arsenal, but will be quick to backstab Korea and America like they had always been doing once they get a technological edge they have completely copied from the Americans by fawning to them their secrets

The trick is to allow Russian Far East and Mongolia to develop in the same route so Japan has more adversaries to contend with and balance. Japan has to be forced to become a valuable ally, or they can seek the path of their own slow internal destruction