https://www.bbc.com/news/world-asia-33755182
This enormous cultural destruction was averted because the US Secretary of War had taken his honeymoon in Kyoto decades earlier. Presumably he had no such emotional connection to the innocents killed in Nagasaki.
Esp since civilians were the main casualties and they could afford to shop around for locations.
The Japanese had sufficient time to surrender after the first nuke was dropped.
The second was apparently what was required for them to capitulate. The loss of life (on both sides) from an invasion would have been at least an order of magnitude greater.
One terrible action to avoid a prolonged much larger loss of life. I don't envy the people who had to make that difficult decision but it is clear they made the correct one.
Esp since both nukes and fire bombings did far more damage to civilians structures at that point.
Even if you ignore the wanton destruction, Russia flipping over to allies and cornering Japan by that time would have been enough to destroy Japanese moral as much as the fire bombings did.
So really the excess use of force by that time wasn't really needed, and trying to justify it now sounds like making poor excuses at revisionism.
Imagine you’re Truman or a General who has watched countless Allied and Japanese die in the war. How exactly would you get an enemy like that to surrender if you’re not willing to deliver a couple knockout punches?
Like WTF.