First, the first 2 nukes don't make Japan surrender. Emperor is a nut case and declares that he will fight on. Numerous extra bombs are loosed on Kokura, Tokyo, and other cities. Japanese population slaughtered.
Next, the Korean War is turned into the Japanese War when Communist China and Korea (unified by USSR) invade West Japan. Population slaughtered again.
Finally, the war is won by USA and allies. Numerous USA settlers come to Japan to establish thriving businesses, and the islands are taken in a manner similar to the annexation of Hawaii, because American settlers become a majority or significant minority.
Sounds ridiculous and ASB, but so does the idea of Japan as a state.