Why the Soviet Union might attack Japan instead

Could a German-Soviet invasion of Poland widen the Tsingtao war into an alternate Second World War while Britain remained focused on Japan?

The Soviet Union might find it advantageous to join the coalition against Japan, leaving Germany more isolated than the proposed German-Soviet alignment assumes.

The comment imagines Germany and the Soviet Union invading Poland while Britain, France, Italy, and eventually the United States are already engaged against Japan. It asks whether France would be left on the defensive, how Italy and Turkey would react, and what the arrangement would mean for Finland, Norway, India, and the Far East. The Soviet Union might instead join the allies in attacking Japan. That possibility changes the alliance map and warns against treating the historical German-Soviet partnership as fixed under radically different Asian conditions.