The Unauthorized History of the Pacific War podcast just did an interview with Richard Frank about the decision to use the atomic bombs. It's on their YouTube channel.
Truman was going in. He approved all the preparation for the invasion of Japan and General Marshall was for the invasion. Admiral King was against the invasion until Admiral Nimtz supported the invasion. I want to point out that all these men grew up in the shadow of the Civil War and lived through WWI. They may not like the idea of causalities and they all knew Lieber's Code
"sharp and brief war" or some say
'sharp war is a just war". They understand Neopolean's quote "Men are not men are my instruments"...
Truman dropped the bomb to save American soldiers' lives and bring the war to an end. The timing was influenced by the Russians because they were planning on invading Japan sooner than us...
"Tokyo rocks under the weight of our bombs...I want the entire world to know that this direction must and will remain - unchanged and unhampered, Our demand has been and it remains - unconditional surrender." President Truman, in his initial address to Congress, 16 April 1945.
But the Army led by General of the Army George C. Marshall believed the critical element was time. A protracted war would squander public support.
Therefore, the Army advocated an invasion of the Japanese Home Islands as the swiftest path to concluding the war.
This link is a good summary of the planning of the invasion of Japan... lead up...
On 8 May 1943 the American Joint Chiefs of Staff (JCS) approved a broad plan for future operations in the war against Japan. Entitled the "Strategic Plan for the Defeat of Japan," the plan aimed at securing control of the South China coast, and the islands of Luzon and Formosa. These areas were needed to develop air bases from which to bomb Japan and sever her supply lines.(1) The memorandum also stated, "The United Nations war objective is unconditional surrender of the Axis Powers. The accomplishment of this objective may require the invasion of Japan." This is the earliest official mention of the possibility of an invasion of the Japanese homeland.(2) There were two principal commanders with the responsibility of executing U.S. war plans in the Pacific. General Douglas MacArthur, USA, as Commander-in-Chief Southwest Pacific Area (CINCSWPA), operating in the South Pacific, and Admiral Chester Nimitz, USN, as Commander-in-Chief Pacific Ocean Areas (CINCPOA), operating in the Central Pacific. General MacArthur's campaign in New Guinea, and Admiral Nimitz's drive across the Pacific to take the Caroline and Marshall islands during 1943 and early 1944 were steps in the strategic plan to take Luzon, Formosa and the South China coast. However, throughout the summer and autumn of 1944 military planners in Washington argued over the timetable and priority of the Luzon and Formosa operations. There were not enough troops or ships in the Pacific to conduct both operations simultaneously. On 3 October 1944 the Joint Chiefs of Staff decided on the priorities of the strategic plan. They directed General MacArthur to invade Luzon and dropped Formosa as a target in the plan.(3) Instead of an assault on Formosa, Admiral Nimitz was directed to secure the islands of Iwo Jima and Okinawa as advance bases in support of the air campaign against Japan.(4)