There have always been those who disagreed, including Ike, George Marshall, Wm. Leahy, Curtis LeMay and IIRC Nimitz and MacArthur.
I have never heard that, please give some specific links that shows your claim to be true. Thanks.
Eisenhower and Leahy: https://www.washingtonexaminer.com/it-wasnt-necessary-to-hit-them-with-that-awful-thing-why-dropping-the-a-bombs-was-wrong?_amp=true
MacArthur, Hap Arnold, LeMay, Ernest King, Bull Halsey, Nimitz: https://www.nationalww2museum.org/war/articles/atomic-bombings-ian-w-toll
These were serious men and there were and are serious men who thought the opposite. I am not condemning anyone.