Used on military property. Military targets are legitimate targets in wartime. The Japs had no problem bombing our military property in peacetime, why should we feel guilty that we did so during wartime?
They weren’t military properties; they were selected because the few planes Japan had left wouldn’t be defending them.
Those two cities would be considered military targets within the meaning of the Third Geneva Convention based on the then ability to precision bomb, but further discussion would require an essay. The short version is precision bombing (?) generally meant putting 10%-20% of bombs on the target. Also, wartime Japan relied on a host of surrounding cottage industries to keep their plants going.