Let’s say Stalin gets the idea in his head that a-bombs could be a decisive weapon. He put the considerable resources of the Soviet command economy to work on an urgent basis to make him some. How long would it take before the Soviets had an A-bomb? How much would that effort take away from other Soviet military production? Fewer tanks? Fewer planes? Fewer artillery pieces? What impact does that have on the war?
Could the Soviets have beaten the US to having an A-bomb, or been able to match the US blasts with their own within a month or two? I'm guessing probably not, but the Soviets were capable of pulling off some amazing manufacturing achievements, so I don't rule that out entirely. They would also presumably have access to their spies inside the US program. If the Soviets did get their bomb earlier, how would that change the postwar world?