But the Allies decide not to use their bomb. They are better men than the Soviets, they say.
In any case, they were winning. The Soviets fall back on all fronts, losing what used to be Poland (which was incorporated into the USSR) in literally days. Soon after that, Siberia falls to the forces invading from Japan.
And the Soviets are forced to launch their own nuclear bomb, come August 4th, 1945. Moscow is surrounded- all else has been lost- but the airfield has not yet been bombed.
Thousands of Allied soldiers are wiped out in a single day.
The Allied generals are faced with a choice. There is still one division surrounding Moscow, and if they invaded, they could end the war, once and for all.
Or, they could respond in kind. Nuke Moscow. Push the death total from the war to 30 million.