By the time the nuclear bombs were dropped, it was already clear that Japan had lost the war. In fact, Japan was already willing to conditionally surrender, but as I'm sure you can appreciate, after the horrors of WW2 the allies were interested in nothing less than an unconditional surrender.

Japan was hoping that the Soviets would be able to broker a peace between the two parties that was more favourable to Japan.

The bombing of Nagasaki and the declaration of war by the USSR happened on the same day, and there is considerable debate amongst historians about what ultimately led Japan to finally surrender.

I'm personally in the camp that the Soviet invasion was vastly more impactful than the Atomic bombs, but that's appeared to be somewhat unpopular on reddit in the past.

Mountain ranges/coast lines provide natural boundaries that are better at defending invasion.

Which way a river flows can also have an impact. The Alps to the north west and the Danube flowing east had a pretty big impact on Austria's ultimate sphere of influence, for example.