Effects and Reactions to a Soviet Pre-Emptive Strike?

What would be the effects and reactions if the Soviet Union were to attack Germany in late May 1941? I'll let you decide the outcome of this event but what would be the political, social and economic impact in places like America and Britain as well as the other great powers and the rest of the World.
 
Everybody in the war would have a predictable reaction.

Britain would be happy.

Germany and Italy, wouldn't.

The interesting one would be the US. It would probably strength the hand of the isolationists, since they would certainly argue that the Nazis and Bolsheviks were just as bad as each other.

If the Soviet's attack doesn't goes well, it doesn't matter too much. But if the Nazis beat back the attack and invade the USSR back, the long-term consequences could be dire for the allies - perhaps cut-back in lend-lease, etc. (Also it might be harder for Stalin and co to rally the Russian patriotic spirit in this situation, if the Communists started the war).

It also affects Japan too. If Japan sees the Russian attack succeed, it will fear Russia more, which logically should make her more cautious, but because Japanese policy is so completely mad anyway, we don't know that for sure. If Japan sees the Russian attack fail, she might fancy a go (especially this will be in the wake of the Russian poor performance against Finland), logically the Japanese might see an opportunity to move against Russia then - but again Japanese policy is so completely mad anyway, we don't know that for sure.
 
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