It is relatively well known that Macarthur was against using the atomic bombs on Japan for a variety of reasons. The first was that he felt that the atomic bombs should be kept for use against the rising communist tide, something he would repeat during the conflict in Korea as he wanted to use them against the Chinese.
The second was the fact that he was familiar with the state of Japan towards the end of the war and did not feel it necessary to use the bomb on them as he felt they were trying to surrender but it would take more time. He also knew he was slated to take over as the governor of the country during its occupation and feared reprisals after the use of such a weapon.
His 40-page memorandum to President Roosevelt was verified several times by historians and later by Macarthur himself when it was brought to him to see if it was in fact authentic.