Given that Japanese military communications remained open to US code breakers right up to the end of WWII, did the US top brass realize how well prepared the IJA was for an invasion of Kyushu? Conservative estimates for American casualties topped half a million, so if American leadership also has a detailed breakdown of KETSU-GO (the Japanese plan for the defense of Kyushu) how likely is DOWNFALL to be scrapped? Would the alternative be to simply starve the Japanese into submission, or would atom bombs have been dropped on Japan as fast as they could be manufactured?
That was actually something that was starting to come to a head in August 1945. Magic and Ultra intercepts, confirmed by photo reconnaissance, revealed the sheer scale of Japanese defensive preparations on Kyushu during the course of July and despite MacArthur's best attempts to suppress it, that intelligence had managed to make it's way to the Navy. As early as May of 1945 Nimitz was expressing the view that Operation Olympic (the planned invasion of Kyushu) was unwise, having advised King on May 25 that he no longer supported Olympic. As the intelligence continued to gather showing massive Japanese forces building in Kyushu many other officers echoed these doubts. When King received a report by MacArthur on 9 August claiming that the intelligence was grossly exaggerated (it was not) and that Olympic was still viable, he immediately forwarded to Nimitz asking for him to append his own comments and then forward the whole thing back to MacArthur. Since King already knew that Nimitz opposed the invasion, Frank is certainly correct in judging that King's intent was to "bring on an explosive interservice confrontation over Olympic." [Richard Frank, Downfall p.276]
Similarly the US Army Air Force was also poised to come out against the invasion. On 11 August, following the receipt of a comprehensive study on the effects of bombing in Germany, Spaatz had issued a new targeting directive turning the bombers away from firebombing of population centres and instead aiming them at Japan's transportation network. That same day Spaatz also wrote in his diary that he intended to write the Assistant Secretary of War, arguing that the invasion should be cancelled so the atomic bombs and the newly retargeted bombers could be allowed to do the job.[p.307]
The Joint Chiefs also were not solidly committed to an invasion. On the 9 August when the first bomb was dropped on Hiroshima, the Joint Plans Committee of the Joint Chiefs forwarded a report titled "Alternatives to OLYMPIC" which asked field commanders to review their own plans in light of the Japanese build up on Kyushu and prepare plans for alternative operations. At the same time, King was also revisiting the concept of Olympic as he had previously warned the Joint Chiefs he would do in August. MacArthur on the other hand was gung-ho for staying the course of Olympic and played down the intelligence, claiming it was grossly exaggerated. MacArthur was known to exaggerate himself when he wanted his way and among senior American officers his judgement was known to be suspect.
Ultimately, had the war persisted then by late August Truman was likely going to be presented with dissenting arguments from both the USAAF and the USN against an invasion, as well as a JCS that was becoming somewhat wobbly on the whole idea as well. While Truman wanted the war over, the idea that he would have forced a costly plan for its conclusion in the face of opposition from the Navy and the Army Air Force, particularly when they casualties of that invasion could have doubled the American war-dead, is not very credible. And while a longer war might not have been desired, it was certainly not beyond America's capability to fight.
All of this brewing drama was nipped in the bud by the Japanese surrender but Frank lays out a very clear argument that as of early August the consensus agreement on Olympic and invasion was swiftly unraveling.