Hiroshima and Nagasaki were not “civilian targets”. Both cities were military manufacturing centers. That’s one of the reasons they were chosen.The U.S. didn’t know the long term results of nuclear fallout when the bombs were dropped. No one did. The U.S was the FIRST to have a working nuclear device, with Germany and Japan closing in on having one when the war ended for each.
Evidently, the U.S. nuclear policy works and has worked for more than 71 years. No one else has used one.