Considering the US government's low opinion of General MacArthur* even before he almost performed a military coup in the 1950s:
Was Douglas MacArthur ever made aware of the Manhattan Project before Little Boy was dropped Hiroshima?
And if so, when?
* And frankly my own low opinion of the guy as possibly the most incompetent career general in a position of power that America has had this side of the Civil War.
It appears that he was informed before the bombings, but only a few days before. However, this was probably more a matter of information security doctrine than of any lack of trust in the General personally.
Access to US intelligence information is based on two principles: Clearance level, and Need to Know. In order to have access to a military secret, a person must have both a clearance level at least as high as the level of the secret, and some need to know that secret. Clearly a General is going to have top-level clearance, but he or she might not always have a need to know.
At the time of the Manhattan Project, McArthur was the Commander of US Army Forces in the Far East, and then later at the very end (after the reconquest of the Philippines) he became Commander in Chief of all Army and Air Force units in the Pacific. However, there was one notable exception: the 20th Air Force. Perhaps this is because the 20th was coordinating with the Navy, not the Army. However, this is notable because the 509th Composite, the unit tasked with the "delivery" of the Atomic bombs was organized under the 20th.
We know the Manhattan Project was indeed being kept according to some variant of these principles, as FDR's own VP was unaware of it until after FDR died and he ascended to the Presidency.
The project itself seems to have fallen under the aegis of the Army Corp of Engineers, which were not under McArthur's command. It appears the chief of the US Air Force was breifed on the project by March 1944, as that's the date they had a meeting to discuss "delivering" the bombs.
In the absence of any specific information to the contrary, it would probably be reasonable to assume proper information security was being followed. In that case, General Macarthur would not have needed to know about the bombs before he became commander of AFPAC in April of 1945. One could argue that he still did not need to know, as knowledge of the bombings was clearly not to influnce the planning for Downfall, which was his primary responsibility, and none of the military units involved in the bombing were under his command.
Now as for specific information on MacArthur's exact date of breifing, I did dig up one article from a historian claiming that he was briefed only days before.
When first informed about their imminent use only days before Hiroshima, MacArthur responded with a lecture on the future of atomic warfare and even after Hiroshima strongly recommended that the invasion go forward. Nimitz, from whose jurisdiction the atomic strikes would be launched, was notified in early 1945.
("Early 1945" being February, just prior to the reorganization I mentioned above)
MacArthur's defense of the Philippines on Dec 7 was terrible. Unlike Pearl Harbor, which was thought to be un-attackable at that time (until the IJN showed what could be done with carrier forces), MacArthur's command was well within range of conventional Japanese forces and the likely target of a Japanese attack. However, at that early stage of the war, the US needed heroes, not losers, so MacArthur was 'rehabilitated' as a hero, and made the US commander of army forces in the Pacific, while Admiral Kimmel, the commander of Pearl Harbor, fell on his sword and took the blame.
In all fairness, most of the US was unprepared, physically and psychologically, for the war.
Your charge that he was out of favor with the US government has no basis in fact. Two offensives were contemplated to cut Japan off from its supply of oil, Formosa (favored by Nimitz) and the Philippines (favored by MacArthur). Roosevelt chose MacArthur's plan, so he certainly couldn't have been out of favor in the 1944-1945 period when the atomic bomb was made reality.
If MacArthur wasn't informed about the atomic bomb until shortly before its use, it is probably because he had no need to know, because his forces were nowhere near where the bomb would be used, while Nimitz had quite a few forces in the general vicinity of some of the potential targets, subject to potential fallout. The Manhattan project operated on a strictly need to know basis. This was not a sign of favor or disfavor, more an indication of just how secret the project was kept.
MacArthur was not dismissed for 'plotting a coup'. He was dismissed for publicly contradicting the commander in chief. That is the fate of any high ranking general who is seen to criticize their chain of command. Stanley McCrystal, a brilliant general who had done very well in Afghanistan, was dismissed when private comments he made disparaging Obama were leaked.
Trivia story: during WW1, a US unit was subject to a particularly intense German shelling. Two officers did not take cover, staring each other down to see who would flinch first. Neither did, and both survived unharmed, by luck. One of the two officers was Douglas MacArthur. The other was George Patton.
Indeed MacArthur was made aware of the atomic bombing(s) only days beforehand and was "upset" that both Eisenhower and Nimitz were briefed well before. I agree that it was a simple formality and not a security matter, but what is puzzling is that when the decision was made to drop the bombs, and presumably "end the war", soldiers in MacArthurs Army were still engaged and fighting. It seems to reason that had he been informed earlier than he was, American lives could have been saved by less engagement with hostile forces.
