From The Accidental President: Harry S. Truman and the Four Months That Changed the World, by A. J. Baime (HMH Books, 2017), Kindle pp. 248-250:
The brain trust of the American military gathered. Here sat General George Marshall, Fleet Admiral Ernest King, Lieutenant General I. C. Eaker of the army air forces (representing General Arnold, recovering from a heart attack), and the chief of the president’s staff, Fleet Admiral Leahy. Secretary of War Stimson was in the room, as were Assistant Secretary of War John J. McCloy and Secretary of the Navy Forrestal. The president wanted to know from each an opinion on the most efficient means of forcing Japan to surrender unconditionally, and to bring the war to an end.
General Marshall spoke first, reiterating arguments he had already posed but now with more detail. The situation in Japan was “practically identical” to the situation in Europe before the Normandy invasion, Marshall said. He believed that “the only course to pursue” with respect to Japan was the course that had brought the Nazis to their knees: a ground invasion. He had chosen the island of Kyushu at the southern end of Japan’s mainland for the landing, and he set D-day at November 1—four and a half months’ time.
Marshall listed the reasons for the timing: “Our estimates are that our air action will have smashed practically every industrial target worth hitting in Japan as well as destroying huge areas in Jap cities,” he said. “The Japanese Navy, if any still exists, will be completely powerless. Our sea action and air power will have cut Jap reinforcement capabilities from the mainland to negligible proportions.” Any delay past November 1 could force a further delay of up to six months due to winter weather, he explained.
The general then discussed what could be expected in casualties. The United States had suffered roughly 20,000 casualties (killed, wounded, missing) in the invasion of Iwo Jima, against an estimated 25,000 Japanese (killed and taken prisoner, for there was no way to even guess how many were wounded). In Okinawa—the fiercest fought ground battle of the Far East war, and one in which the U.S. forces were on the brink of declaring victory—the Americans had suffered 34,000 army and 7,700 navy casualties, against 81,000 Japanese (the latter number being “not a complete count,” according to the military statisticians). U.S. casualties in the first thirty days of the Normandy invasion had been 42,000. There was no way to estimate the number of casualties expected in the invasion of mainland Japan, but Marshall did say this: “It is a grim fact that there is not an easy, bloodless way to victory in war and it is the thankless task of the leaders to maintain their firm outward front which holds the resolution of their subordinates.”
Marshall was convinced that “every individual moving to the Pacific should be indoctrinated with a firm determination to see [the invasion] through.” He put the number of troops required for the operation at 766,700. The invasion plan was as follows: (1) to have the Russians attack the Japanese occupying Manchuria in China; (2) to “vitalize the Chinese” with air support and supplies so they could handle the Japanese occupying other parts of their country; and (3) all of which would allow the Americans—with British aid—to go after mainland Japan.
Truman went around the room and heard not a single dissent.