The casualty toll at Okinawa pretty much gave a bleak outlook for the invasion of mainland Japan. The fierce resistance and the fanaticism of the civilian population as well as the Imperial Army caused the projected casualties for the invasion of Japan to be in the millions. The Battle of Okinawa helped the decision to use the atom bomb against Japan seem the less bloody alternative to defeat Imperial Japan.
Very true. What people forget about when they say the bomb was racist or dropped to impress the Soviets is the US had just fought Iwo Jima and Okinawa. Both were the worst battles of the Pacific war. Each had the characteristics of a mini-Stalingrad. Dug in armies attacking the same ground over and over.
The US had something like 200K in land troops at Okinawa with a KIA rate of roughly 3.5%. But the Japanese only had about 110K against them. When the US intended the invasion of Japan, the Japanese considered this the opportunity to bring their Ground Forces into the battle. They were looking at getting over a million troops into the fight. Whereas the US planned for 325K defenders, there were, in actuality, over 820K on Kyushu by the time August of 1945 rolled around.
In the invasion of Japan, If we take the Allied losses as a percentage of Allied troops involved and use Okinawa as the yards stick, we have the following:
550K x .035= 19,250 dead.
Further, if we include total casualties (Okinawa had 72K total non-fatal casualties or 36%), again using Okinawa for the basis, we get the following:
550K x .36= 198,000. That would be broken down into Wounded, Missing, Sick, Psychological Casualties (a large and getting larger segment of non-battle casualties).
Total Allied losses for the invasion of Japan using the US operation at Okinawa as a template, 217,250 is the number. Roughly 39.5% of the force.
Japanese Casualities, using the same method are as follows:
Okinawa total force: 120K
Kyushu: 900K (using an average of the August number of 820 and the probably number of November 1st, 1 million. Just averaging down on it for conservative numbers sake)
Total number of Japanese troops on Okinawa: 120K
KIA on Okinawa for Japanese troops: 110K,
Captured: rougly 8-10K (first battle in the pacific where significant numbers of japanese were captured)
Japanese (military) for the invasion of Japan:
900K x .91= 825K KIA
Captured: 81K
Japanese Civilian casualties would've been signicantly worse than Okinawa as months of poor food and hard work coupled with shortages in same foodstuffs and poor medical care/hygene increases the number. On top of that, the rice crop failed in the fall of 1945. The civilians would not be moved from the battle areas and would be expected to fight and die.
My offhand guess is something on the order of 2 Million dead in the Civilian end. That covers all of Japan. Most of that is caused by disease and starvation with the old and young be the preponderance of the dead.
So, by putting it all together (and leaving out the dead Allied POWs and civilians in Japanese occupied areas of China and Asia), we have something on the order of 3 million dead in an invasion of Japan.
Without a doubt, it is the bloodiest land battle never fought.