Comment by NateEag

The destruction of Tokyo, Dresden, and so many other cities were also unconscionable acts. The war was absolutely full of them (though I think the Axis was clearly aiming for unconscionable overall goals, such as the Final Solution, so I still argue the Allies were fighting a just war, even if they often carried it out through unjust means).

The nuclear bombings were what the OP brought up, so I was just registering my opinion of those.

With the nukes in particular, there were a lot of other options, as they were announcing weapons that would completely change the balance of warfare.

I think a good first step would have been to find an uninhabited island and explicitly tell Japan through diplomatic channels, "We are going to destroy this island entirely with a new weapon we have developed. Please observe, and surrender afterwards if you do not want your military force to be annihilated by these means."

Then Japan would have had a strong reason to back down that didn't involve the US destroying entire cities and civilian population.