Japan and the US are one stock market when seen through the lens of a single currency.
click chart to enlarge
click chart to enlarge
I am embarrassed. Previous charts of this relationship were incorrect. They showed the S&P in euroyen terms, not yen terms. I apologize. The correlation is quite a bit tighter now. The weak dollar versus world currencies had been helpful during the recent rally. A weak dollar versus the yen is not helpful.