We have Truman’s Sec. of War Stimson’s own source document on the dropping of the atomic bombs on Hiroshima and Nagasaki. We also have the War Dept.’s records to refer to. So, no unsupported speculation is required.
The taking of Okinawa was the best warning of what the taking of Japan itself would entail. Okinawa is about 875 square miles, at the time holding 130,000 Japanese troops and 450,000 civilians. The US attacked with 183,000 troops. The Japanese drafted 40,000 civilians to serve as militia.
1,900 Kamikaze planes sank 36 ships and damaged 368 ships, killing 4,907 sailors. Nearly 150,000 civilians were killed in the fighting, nearly one out of three. US ground forces lost 7,373 killed. Japanese military lost 107,000 killed, and another 20,000 whose bodies were never found.
Japan itself was much more heavily defended than Okinawa, but Okinawa gives us a minimum estimate of what to expect from an invasion of Japan.
Japan is 167 times larger than Okinawa. It held 2 million troops, and the civilians were being drilled to fight. There were also 2 million troops in China, most of whom would be transferred to Japan in the lead up to an Allied invasion. The Japanese fought to the death on Okinawa and would certainly have done the same on their homeland.
The Allies could muster around 5 million men, not a great deal more than the defenders. It was estimated that the fighting would last more than a year, and cost a million Allied lives. The Japanese would lose many times that number in military lives, and great numbers of civilians.
Now regarding the use of the two atomic bombs, the principle justification is the lives saved by ending the war quickly. These are relevant facts:
The two bombs killed about 110,000 civilians, by the best estimates; less than died in 82 days of fighting on Okinawa. A single firebombing air raid on Tokyo, by conventional weapons, killed nearly 100,000 civilians. Similar raids were happening in other cities of Japan. If the war had been prolonged, that slaughter would have continued by conventional means, causing many times the civilian deaths as did the atomic bombs.
Furthermore, the Japanese were killing about that same number of civilians, 100,000, every several weeks in China. If the war had been prolonged, those Chinese deaths would have continued.
If the Allies had to defeat Japan by invasion, the civilian deaths from the fighting would have numbered at least ten times that from those atomic bombs. The Japanese military deaths would have been forty times that number. The Allied deaths would have been ten times that number. The Chinese civilian deaths would have been eight to ten times that number.
The arithmetic may be cruel, but in such a war, the kindest thing is to end it as quickly as possible.