General Sweeney in an interview I saw stated when asked why he wrote the book said he wanted to make sure the real story was told before the truth was lost.
You triggered a memory of seeing an interview with him where he uttered those very words. It did not connect until now.
I have been doing a little research, and apparently Stimson was calliing the shots. Supposedly, he had already advised Truman to drop the atomic bombs on Japan, but of interest: Stimson later noted in his 1948 memoirs, written with McGeorge Bundy, "Only on the question of the Emperor did Stimson take, in 1945, a conciliatory view; only on this question did he later believe that history might find that the United States, by its delay in stating its position, had prolonged the war." (Stimson & Bundy, "On Active Service in Peace and War" pg. 628-629).