The A-Bomb got their attention. Also, it prevented the USSR from turning Japan into "North Japan/South Japan".
The sheer magnitude of the invasion of japan under the name “Olympic” was going to make D-Day look tiny.
over a THOUSAND ships.
over 1 million tons in supplies
and the alies casualties were anticipated to be high.
You are correct. It would have been to the death for both sides. It would be no touchy feely introspective hollyweird BS.