We're still giving out Purple Heart medals made for the expected casualties that we would have suffered in an invasion of Japan. Think about that for a moment, the sheer number of killed and wounded we've suffered in Korea, Vietnam, Desert Storm, Mogadishu, Iraq, Afghanistan, and all the other battles along we way and with all those casualties, we've still not exceeded the number of casualties from one expected WW2 campaign.
So even assuming the first bomb was necesary... the second bomb was pure sadism.
Actually it was not. Let me give you the three choices Truman actually had along with the timeline of events.
In February of 1945 the USSR and US officially started the COLD WAR after the Yalta Conference. Look it up.
USSR showed they had no desire to be allies and Stalin was looking into expanding their territory and expanding communism. They began to encroach into the middle east and they had a desire to take over the trade routes of the middle east.
Stalin was declaring his desire to take over the region, even after the Potsdam Conferences in July of 1945.
August 8th, the day before Nagasaki, the USSR had declared war on Japan. On August 9th Nagasaki bomb dropped. USSR backs off and Japan surrenders.
So, if you are Truman, here are the three choices.
1. Allow Stalin to take over the region, allow Japan to surrender to the USSR where Japan through propaganda could save face instead of a humiliating surrender to the US. How would that play out with the American people after the years of the Pacific campaign that cost thousands of American lives, where the USSR did not commit anything to that campaign?
2. Declare war on the USSR like Patton wanted. Logistically it made some sense as Patton said. The US hardware was already there. The US war factories were in good working order. The USSR were very weary after a long drawn out battle with Hitler. However, they were still formidable. Stalin had moved most of their factories east and far away from Hitler's reach. They had a formidable airforce and navy. A war with the USSR would have been devastating and the US would have had to go it alone. The UK would not have joined us. It would have been an almost impossible to sell to the American people.
3. Drop the bombs to force the USSR (Stalin) to back off, or else.
Those are the choices. Explain why the USSR declared war after Hiroshima on August 8th and then backed off after the second bomb was dropped.