Read "Tennozan", a history of the Battle of Okinawa and the subsequent deployment of the Atomic Bomb. The fact is, the Americans had been taking progressively horrific casualties throughout the Pacific War, from Guadalcanal to Tarawa to Peleliu to Iwo Jima to Okinawa. Iwo Jima may have been the bloodiest battle in Marine history, but Okinawa was the absolute bloodiest battle experienced by Americans in the Pacific theatre in WWII and was fought by the US Army. With over 20,000 dead, the casualty rate was so high that President Truman was determined to use the Atomic Bomb to end the war rather than risk any more loss of life likely to occur during an invasion of Japan. You can say that the tenacity and brutality of the Japanese made the use of the Atomic Bomb a wartime necessity.
True. But of greater concern was that the intel was STILL faulty after Iwo and Okinawa: as Richard Frank ("Downfall") shows, in Operation OLYMPIC, we still failed to notice that the Japanese army moved two more divisions to the southern island in anticipation of the invasion, meaning all the casualty estimates---then and now---were way too low. Thank God for the a-bomb.