World War II shattered every great power except the US and Russia. When Germany surrendered, the Soviets had overwhelming military superiority in Europe. Had Stalin chosen that moment to try to overrun the rest of Europe, could he have been stopped? It's hard to imagine how. The much smaller US Army in Europe would have been hard pressed to stop them.
Stalin already knew, though, that the US was close to a nuclear bomb - the Trinity test was only two months away, and after Hiroshima, it was clear that that option was impossible.
I seem to recall that Bertrand Russell suggested that it would be a good idea for the US and Britain to use their nuclear superiority at that point to force Russia to surrender and give up it's plans for a bomb. Many of the scientists at Los Alamos foresaw the world of today, where all sorts of countries would have nukes and the means to deliver them, and thought that the only viable alternative was global control.
Today we are on the brink of a world where dozens* of countries, some led by madmen, have nuclear weapons and the means to deliver them globally.
*The list today likely includes the US, Britain, France, Russia, China (the five official nuclear weapons states), India and Israel. Pakistan and North Korea also have bombs, but likely still lack the means to deliver them globally. Iran is probably not far behind. South Africa formerly had the bomb, and today essentially every technologically advanced state could build them rather quickly. It cost the US about 2 billion dollars to make the first bombs (perhaps 20-30 billion in today's dollars), but today it would be far, far cheaper.