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.
My father was in the Army in 1944, deployed in the Japanese theatre. He was 17 years old
For as long back as I can remember, I heard my father and his buddies, with the help of a few beers, saying, "it is a good thing there was the A-bomb or we would probably not be here. Nor you, young man," as he looked at me.
USMC was also at Okinawa. My uncle was a BAR man and picked up a Purple Heart and Bronze Star there.