“Why should we express regret for saving tens of thousands of American lives?”
Actually the number of Allied losses to take Japan with boots on the ground, the most optimistic estimate it would have cost 250,000 allied casualties with most estimates at around a half million.
We believed the war had to be brought to a swift conclusion because we had incontrovertible evidence that Hitler sent U-boats to Japan with atomic tech and materials, as well as jet engines and fighter plans for Japan’s last ditch defense. We know it for a fact because one of the Uboats was captured, and because Japan rapidly produced their copy of the Me262 as well as rocket kamikaze planes from the Uboat that made it. So we had reason to believe they were at least capable of making a dirty bomb given time.
The Japanese laboriously hollowed out mountains and placed their military manufacturing underground; completely bombproof. It would have produced jet fighters and suicide planes to hammer the invasion fleet and landing forces. Films and photos show them training civilians for mass charges with bamboo spears and teaching children to run under enemy tanks wearing explosive vests. Remember, these people were so indoctrinated they threw themselves off cliffs in Saipan because they believed we were cannibals.
Dropping the atomic bombs ended up saving millions of people.
Depending on the degree to which Japanese civilians would have resisted the invasion, estimates ran up into the millions for Allied casualties.[2]