The justification
at the time for nuking Hiroshima and Nagasaki was field testing two different novel bomb designs .. the urgency came from Germany's surrender and unavailability to use as a test bed and from the rapid depletion of pristine targets in Japan.
The H&N bombings followed close on the heels of bombing 72 other cities (including Tokyo) as part of an ongoing HE + incendiary campaign with list of targets.
These specific targets were selected for atomic tested as they had not been bombed before and served as "clean" test beds for the before and after comparisons, in addition to having some containing topography.
What's important to remember is that they were selected from a long list of targets that were all scheduled to be bombed, the fact that they were low priority from a military standpoint is what had "saved" then from not already having been bombed.
When Hiroshima was bombed the only prior atomic test at Trinity was on a tower with a lot of external controls .. it wasn't even certain at the time that this would work as a bomb let alone "end the war".
The military compulsion to battlefield test a weapon that had consumed more R&D budget than any ever before in history was intense, and the WW's were rapidly closing out with Germany defeated and Russia closing in on Japan.
After the bombings, immediately after, came a lot of retro fitting justifiction, more so with the Cold War .. but it was never as clear cut and about swift endings and saving casualities as came to be believed.
People forget that atomic bomb or not the US was already committed to levelling all cities within Japan.
For more, and a deeper dive in the many takes on dropping the bomb, see:
https://blog.nuclearsecrecy.com/2013/03/08/the-decision-to-u...
for example (it has many references to many historic viewpoints)