So, for a challenge today: how would we get a Communist Japan, and one that wars with the United States?
My first thought would be a fall to Socialism/Communism before World War II, rather than a rising of a Communist Japan that went to war afterwards, because a cold war Communist Japan actively fighting the United States seems a bit far-fetched.
Two possibilities that immediately spring to mind:
1. A longer Russo-Japanese War leads to the same sorts of rioting and mutinies that brought St. Petersburg down.
2. The Rice Riots of 1918 get even more out of control, and some degree of organized rebellion, rather than just the rioting and disorder, takes hold. That probably would require the Socialist and Anarchist movements not to have been heavily suppressed before the First World War.