(News to me about Hoover, thanks!)
Can't speak to the Korean issue then.
Joseph C. Grew was ambassador to Japan for the U.S. at the time of Pearl Harbor. Later as a State Department advisor and I believe, but cannot confirm, an assistant to Truman. At State he was on a committee examining use of the bomb and counseled against its use against Japan and made arguments similar to those of Hoover. In his memoirs he says he made these arguments directly to Truman.
There was a great deal of agitation within our scientific community developing the bomb. A number were adamantly opposed against civilian targets. A demonstration against a small island was rejected as not a powerful enough message. One proposal was using the first bomb on Kyoto which no less than Henry Stimson, Secretary of War, blocked because he had travelled in Japan and recognized its spiritual and cultural significance. A quick look at Wikipedia gleaned some insight that using the bomb against military targets was an idea of Jimmy Byrnes. Truman is reported to have said to the New York times on learning of Hitler's invasion of Russia that after the war we should help whoever lost and worry about whoever won. Given a discrete citation for that by a colleague I was never able to confirm it. (Hate microfilms!!) Byrnes was definitely a hawk on the USSR and from Truman's memoirs relating to the Potsdam meeting of the Big Three it is clear he was clearly upset about what the Russkies were doing in their zone of occupation. When the US delegation learned of the successful first test in Nevada, there was discussion of whether to inform Stalin, which they did. He said, "use it." And then later told Zhukov (parallel to Eisenhower), "let's tell Kurchatov to speed up his work." If I remember correctly Stimson in his memoirs also argued against carrying "this weapon ostentatiously like a pistol on our hip" in dealing with the Russians.
A number of physicists, namely PMS Blackett have argued as he did in Fear, War, and the Bomb, that using it was the first step in the post '45 Cold War. Others note that it began with our antipathy to the Bolshevik Revolution and our invasion of Russia on two fronts during its civil war. (The Siberian expedition was ambiguous as Betty M. Unterberger has pointed out.) The venerable Hanson W. Baldwin, long-time military correspondent for the New York Times also considered dropping the bomb a blunder in his Great Mistakes of the War, especially since MacArthur followed Margaret Meade's advice to rule through the institution of the Emperor. Who says anthropology isn't useful?
More pedantry next time, unless Audie gives me the hook.