Only as much as needed to clean-up a predetermined decision for a show of power for the post-war era to fit the official narrative.
(And even if it was "that a land-based invasion of Japan would be extremely costly for both the invaders and the civilian population" still wouldn't justify nuking civilians. War crimes are not excused when they make one win more cheaply).
>It's pretty terrible as a weapon of mass destruction. Don't you think that if the US had wanted to annihilate the population of North Vietnam, they would have employed something a bit more direct?
Why would they want to annihilate them? They wanted to win the war. So fucking their livelihood and health over AND still getting to use the hypocritical defense of "see, we weren't meaning to annihilate them" works even better when you need to maintain face.
If anything of the kind was done on their own soil of course, they'll still be screaming bloody murder for all eternity (like Pearl Harbor, even though they purposefully provoked Japan for months before it acted).