In purely military terms one wonders why the USA and the Allies did not press their huge advantage with possession of nuclear warheads (when they had no risk of MAD).
Is there a reason why an enduring hegemony across Europe, Russia and Asia before the Soviets developed their deterrent?
If you mean why didn't the US attack the Soviets with nuclear weapons shortly after WW2 ended, well the US only had a small number of nukes then, it would have taken them awhile to build them up. And supposing they did attack with the small number they had, it wouldn't have been sufficient to defeat the USSR, their conventional forces would have stormed into Western Europe.
Far better to give diplomacy a chance. And besides the US underestimated how quickly the USSR could develop its own WMD.
To add to Izzy, the US would be backstabbing their former ally. It would have little support in the world, and more countries would have aligned themselves with the USSR instead. The Soviets would feel backstabbed, and they would have had the support of many other countries, which supported the US in real life.