From When the Shooting Stopped: August 1945, by Barrett Tillman (Osprey, 2022), Kindle pp. 57-58:
While naval air combat carried on unabated, groundwork continued for the ultimate objective, an invasion of Japan. The overall Allied invasion plan, aptly titled Downfall, originally had been discussed at the 1943 Casablanca Conference, calling for a two-phase assault: Operation Olympic against the southern island of Kyushu in November, and Operation Coronet on the main island of Honshu the following March. Both would be enormous undertakings: Olympic involved about 350,000 men in combat units plus a further 125,000 support personnel; Coronet more than half a million. In comparison, the initial D-Day landings in Normandy committed approximately 150,000 Allied troops.
Building the force to invade Japan required a gargantuan combination of planning, coordination, and logistics. Previously, Admiral Ernest King, chief of naval operations, had reportedly quipped, “I don’t know what the hell this ‘logistics’ is that General Marshall is always talking about, but I want some of it.” In fact, the Navy was the essential factor in transferring troops from Europe and the United States. Nearly everything without wings had to go by sea, and so did many aircraft.
By August 1945 at least four armored divisions were based in the continental United States, with two or more infantry divisions preparing to deploy west.
The Army also intended to redeploy more than 395,000 men directly from Europe, all between September and December. They included units dedicated to Olympic or Coronet, representing Army Ground Forces, Air Forces, and Service Forces.
At the same time planning proceeded for 477,000 soldiers and airmen to round out the Coronet order of battle, moving from Europe through “ConUS” to the Pacific between September 1945 and April 1946. That amounted to a total of nearly 875,000 personnel moving halfway around the world in eight months. And that did not count Army, Navy, and Marine Corps personnel already in the Pacific. Nor did the redeployment figures include Doolittle’s Eighth Air Force units transitioning to B-29s with 102,000 aircrew and maintainers, either from Europe or originating in the States. The transport burden was further increased by 75,000 European Theater hospital patients beginning in late 1944.
Despite the clear logistical nightmare of such an undertaking, there was one clear advantage to the Allies. Throughout the war they had consistently outperformed the Axis in the crucial realm of supply, which was far more than simply building “stuff.” King’s quip concealed the Anglo-American mastery of the logistical trilogy: planning, production, and distribution. British historian Richard Overy properly noted that the American “tooth to tail” ratio of warfighters to rear-echelon and support personnel ran 18 to one; Japan operated at a support to combat ratio of a mere one to one.