Casualty rates at the Battle of Okinawa and Iwo Jima plays a small role in the justification of dropping the atomic bomb, and is by no means the only justification or even the major justification in using the atomic bomb. They were, after all, only used as a statistical report through the casualty per sq. mile report of what can occur when invading the mainland. So yes, it may be a means towards the justification, but as the justification itself it is not.
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