Hans Bethe (Nobel 1967) told me two things I never forgot. (...) Hiroshima may, or may not, have been justified. But in his mind, Nagasaki was without a doubt a war crime. gosint.wordpress.com/2018/08…
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Are you certain it wasn't just retaliation against Pearl Harbor.
Japan was already defeated. Dropping two 20 KT yield A Bombs on Japan (one of each prototype) and killing tens of thousands of innocent civilians to 'demonstrate US power' to Russia constituted a war crime. Unbeknownst to Truman, the Russians had compromised Manhattan Project.
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