See, the japanese are going to reinstate their ninja program. The'll send people into other countries of the world to kidnap all of the healthiest babies, leaving those nations with the sick and mentally handicapped.
They will raise these kids to be pirates, contructing massive ships after 20 years of training them. The pirates will ransack entire nations that are defenseless because they have no able bodied soldiers.
Japan will hold the monopoly on food, gold, and everything of value. America and Russia will sell them their nukes in order to some of it. This will lead to Japan nuking countries they deem not worthy.
In a last ditch effort to destory the Japan/pirate alliance, the entire world will make one large army and overwhelm the island nation. Japan will end up nuking their own people to prevent being taken over.
Then there will be nothing left of earth but a few hundred starved and irradiated humans clinging to what's left of earth, until after 40 years they simply die out.