Without nukes an invasion of japan would killed or wounded more than 200,000 American soldiers
Oh, it would have been many more than that.
One thing that I do not trust is the "casualty estimates" made for the expected Invasion of Japan. They were almost all based upon statistics and figures from the battles in Europe. Where culturally an army would try to retreat or surrender if things were hopeless. As the militaries in Europe had a long tradition of (normally) fair treatment and repatriation after the war was concluded.
Japan however was a different culture, where fighting to the death was the accepted outcome. And even barbarism like the slaughtering of all captured was accepted as what the victors did. Like after over 2 years of captivity essentially as slaves, the remaining 93 civilian prisoners of war on Wake were taken to a beach and machine gunned when their captors thought the island was going to be invaded soon.
And even Secretary of War Henry Stimson was questioning them, in light of the recently concluded Battle of Okinawa (as well as others ranging from Saipan and Iwo Jima to Guadalcanal and Tarawa). On most battlefields against the Japanese, the Japanese deaths ranged from 80% to over 90% (of the over 2,700 military and civilians on Tarawa, only 148 were captured, on Iwo Jima only 216 of around 21,000 were captured). So he tasked future Nobel Prize winning mathematician William Shockley to conduct his own study into what could be expected.
And what Shockley reported based primarily on the casualties from Okinawa shocked those who read it.
The entire War in Europe from D-Day to VE Day saw 766,294 Allied casualties. The Invasion of Japan he estimated would see from between 1.7 to 4 million casualties. Of those, around 700,000 fatalities. And among the civilian population of Japan, from 5 to 10 million fatalities. And take from 2 to 4 years
The Battle of Okinawa compared to the Invasion of Japan would have just been a warm-up skirmish. Yet, that one battle saw close to 80,000 casualties. Shockley simply confirmed what Secretary Stimson already feared. That the estimates he was getting from MacArthur and the others were completely wrong. Especially when MacArthur was expecting "23,000 casualties in the first 30 days", which is 1/3 less than the actual number on Okinawa. And we know now that that was based on an estimate of 300,000 Japanese soldiers on Kyushu. While the actual number was over 900,000.
And Admiral King had estimated 30 day casualty numbers of around 35-40,000. Once again, frightfully low but consider what that would mean. 2 to 4 more years of war, with as many as 40,000 dying every month. That is 4 times the numbers wounded and killed in Normandy. Every month for years.
And most estimates also stated that the "Conquest of Japan" would take an additional 2-4 years. The country and the world were already tired of war, and just wanted it over.