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Am I correct in assuming that Truman and only Truman had the authority to order the bombing of Hiroshima? After all, it was the very first A-bomb and procedures may not have been formalized yet. It occurs to me that maybe a theater commander like Gen. MacArthur could have given the order.

Given that Truman was at Potsdam when he made the decision to bomb Hiroshima, did he have a secure method of communications to use to communicate his order from Potsdam? If so, what was that method? When was that order actually given? I'm guessing it was at least several days before the bombing since someone had to order the bomb moved to Tinian and a plane and flight crew chosen and assembled at Tinian.

Could Truman have aborted if he'd changed his mind? How late could he have successfully aborted? Right up until the bombardier pressed the "Bombs Away" button? Or would it have been early, say up until the moment Enola Gay left Tinian, after which it might have been running silent?

I realize this is all academic now but it's not impossible to imagine Truman having a change of heart and deciding not to bomb Hiroshima. Let's say the Japanese surrendered on August 4 because they could see that the war was lost. I'm curious how late he could abort the bombing if he'd really wanted to.

Truman retained mission authorization within the Oval Office, but all further details were delegated through the normal chain of command to Colonel Tibbets to plan the mission.

Once the Japanese response to the Potsdam declaration, Mokusatsu, had been sent and received, it would have been tacitly understood all around that any further decisions could only be implemented on a best efforts basis. Mission authorization was given and Tibbets arranges a suitable date according to weather forecasts.

Aboard the Enola Gay the bomb was armed after take off and its safety devices removed about 30 minutes before it was dropped. Conceivably that latter process could be reversed, but I am unsure that it would have been attempted. The bomb could always have been dropped at sea for disposal, though General Groves would likely have been most disappointed at that result.

Note that any last minute communication from the Japanese would have been received with extreme skepticism, as merely a delaying tactic. It is unlikely to my mind that any such would have had the slightest bearing on the Hiroshima mission.

The key point here is that there were no special go/no-go arrangements for the mission. Once authorized it was expected to take place at the earliest suitable opportunity. No arrangement for recall subsequent to launch was planned for because any abort decision was up to the commander of the mission, Colonel Tibbets.

I came across a document in the Truman library while researching another question here, which seems to be the actual order by Truman authorizing the bomb drop:

First is the request for authorization, dated 31 July, 1945:

And on the backside is Trumans hand written response: