The United States should have dropped the atomic bomb due to Truman having very little choice.
Few actions in the were were justifiable. Truman had very little choice because he was faced with the Japanese refusal to surrender. This decision was based of the ( estimated ) half a million allied casualties likely to be caused by invading homeland islands of Japan. Also, there were likely death tests from starvation for Civilians as the war dragged on well into 1946