Well, the most plausible way that Japan could become Communist is in World War Two. The USA does not bomb Hiroshima and/or Nagasaki, and decides to invade. The USSR, following up on it's promise made during The Yalta Conference, invades from the north. We end up with a similar situation like in Germany. Communist North, Democratic South. Also Tokyo would be devided into North and South like Berlin was east and west. However unlike Berlin, the communist half of the city would be surounded by the democratic nation.