They are only fighting in Japan. Of course Japan is, in a sense, these people's whole world, but it would still be an exaggeration, for the time being, to say the dude wants to destroy the whole world. It's quite evident they have been buying weapons from the West all the time, so it's not like the West wouldn't know about the situation in Japan. It wouldn't be quite that easy for the scumbag prince to dream of the whole planet. Most likely any ship trying to leave Japan would be sunk no questions asked.