I think in a situation where the Americans didn't build an A bomb then no one would have for some time.
Would it have made a difference to our world now? It's hard to tell, but the Soviets certainly would have taken more Japanese territory than they did in our timeline.
My opinion was Stalin was too pragmatic to start WWIII, Before and after WWII he only provided limited support to Soviet-aligned groups (see: post-war France and the Spanish or Greek civil wars) in other part of the world to avoid antagonizing the west too much.
The real answer is, who knows?
Check out Oliver Stone's Untold History [1], or many other history sources for various opinions. There is considerable debate about it.[2]
In reality, the Japanese were on the verge of surrender. The U.S. had badly firebombed Japan, burning hundreds of thousands of people.
[1] https://en.wikipedia.org/wiki/The_Untold_History_of_the_Unit...
I don't think there was much chance of Japanese surrender without an actual invasion
I don't know about you, but I'd only have to be nuked once before I'd run up the proverbial white flag.
Also it saved a lot of prisoners of war from a cruel death. https://en.wikipedia.org/wiki/Prisoner_of_war#Empire_of_Japa...
I tend to blame human nature for the destruction of our own selves, not the weapons we do it with. One way another, humans will kill each other, nuclear or not.