This one is one of my favorites what-ifs in history: according to the records, had President Truman chosen not to drop the nuclear bombs, the other option was an earth invasion with over one hundred thousand american soldiers landing in Okinawa with orders to take the land and use it as a base to move forward to Tokyo at any cost.
Japan army back then, blinded by their pride and suicidal beliefs, handed short swords to every Okinawan citizen regardless of gender or age and gave them crash courses of how to use them to kill soldiers. These people were mostly women and children who at this point were starved from the famine and very sick, yet they warned of the rape and torture the Americans would inflict them if they ever get close and threatened with death should they not fight back.
So, with those two facts, many historians argue that, while the dropping of the nuclear bombs were a horrific incident that killed so many innocent people, it's possible that the alternative might've caused an even more gruesome genocide, uncountable losses on both sides and possibly the complete destruction of the nation of Japan.