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Assume that some sort of highly unprobable event (maybe Harry Truman had a heart attack and Alben W. Barkley decided to trust UN forces Commander) allowed Douglas MacArthur to use nuclear devices against both North Korea and China (Manchuria). Lets say its a January 1951 and American strategic bombers are going to bomb their targets within next few hours.
How will Soviet Union response? They had tested their own nuclear warhead just few months before outbreak of the Korean War. After the bombings Stalin will see that Americans are not bluffing about nukes and that they would use it if necessary. Another blockade of Berlin would be more possible, IMHO.
How would the rest of the world response? US had just used nuclear power to defeat their enemy twice within decade. Maybe some sort of anti-nuclear movement strenghtening?
What would Mao do? All-out war against Western Allies or something else?
Would incumbent President be reelected after that?