Reinforcing what others have said:
The first nukes were physicist's gadgets, not mass production munitions, and they were shipped unassembled. In fact, the last piece didn't go in until the delivery aircraft of each was airborne. (Navy ordinance officer Parsons flew on Enola Gay for the purpose of arming Little Boy.) No chance of either going off accidentally other than in the air on the way to Japan (which might make for an interesting scenario in itself.)
MacArthur was determined to lead an invasion of Japan. But intelligence on what he was up against was so unpromising that Marshall and the Joint Chiefs might well have pulled the plug on the invasion. If so, the next step of the strategic bombing campaign was destruction of Japan's rail system (the coastal shipping had already been put out of action by aerial mines.) Estimates are that destruction of the rail system would have so dislocated rice distribution that ten million Japanese would be dead of starvation by the end of 1946. It is very unlikely the war would have gone on that long.
The only way the war was likely to go past 1945, in fact, would be if the Emperor was successfully kidnapped and taken into a mountain redoubt "for his own safety", as some Army officers were apparently contemplating. I am dubious they could have pulled it off.
One very unlikely but not completely impossible scenario: Enola Gay suffers an engine failure on takeoff, a not uncommon occurrence with the B-29. Tibbets has orders to ditch in the water rather than risk crashing on Tinian and does so successfully (he was a brilliant pilot), but the aircraft and its precious cargo sink in deep water. As a result of this reversal of fortune (a half-billion-dollar gun bomb lost as an operational casualty), the implosion bomb is held in reserve.
MacArthur succeeds in getting the invasion to go forward, in part by pointing at the Iron Curtain already going down over Europe and convincing the JCS, over their better judgement, that we have to beat the Russians into Japan. Perhaps there's intelligence from some unexpected source (Venona? Hey, that would be cool ...) showing Russian preparations and a real possibility of a Russian force landing in weakly-defended Hokkaido.
MacArthur is watching the invasion force approach the landing beaches from Bataan, his personal B-17. This is rammed by a suicide aircraft, killing all on board. Nimitz, as next senior commander in the Pacific, takes over but it really is too late to call off the invasion -- the political consequences of turning around when the invasion fleet has already been sighted by the Japanese are unthinkable. It takes some time for Nimitz to get a grip on the situation, which is rapidly deteriorating since the defenses are just as awful as Intelligence predicted (which is much worse than MacArthur allowed himself or his planners to believe.) A big scandal is already brewing over MacArthur's grossly optimistic preinvasion estimates...
Heck, I'm not going to write this whole thing for you. You take it from there. (Will be happy to provide my technical $0.02.)
The first nukes were physicist's gadgets, not mass production munitions, and they were shipped unassembled. In fact, the last piece didn't go in until the delivery aircraft of each was airborne. (Navy ordinance officer Parsons flew on Enola Gay for the purpose of arming Little Boy.) No chance of either going off accidentally other than in the air on the way to Japan (which might make for an interesting scenario in itself.)
MacArthur was determined to lead an invasion of Japan. But intelligence on what he was up against was so unpromising that Marshall and the Joint Chiefs might well have pulled the plug on the invasion. If so, the next step of the strategic bombing campaign was destruction of Japan's rail system (the coastal shipping had already been put out of action by aerial mines.) Estimates are that destruction of the rail system would have so dislocated rice distribution that ten million Japanese would be dead of starvation by the end of 1946. It is very unlikely the war would have gone on that long.
The only way the war was likely to go past 1945, in fact, would be if the Emperor was successfully kidnapped and taken into a mountain redoubt "for his own safety", as some Army officers were apparently contemplating. I am dubious they could have pulled it off.
One very unlikely but not completely impossible scenario: Enola Gay suffers an engine failure on takeoff, a not uncommon occurrence with the B-29. Tibbets has orders to ditch in the water rather than risk crashing on Tinian and does so successfully (he was a brilliant pilot), but the aircraft and its precious cargo sink in deep water. As a result of this reversal of fortune (a half-billion-dollar gun bomb lost as an operational casualty), the implosion bomb is held in reserve.
MacArthur succeeds in getting the invasion to go forward, in part by pointing at the Iron Curtain already going down over Europe and convincing the JCS, over their better judgement, that we have to beat the Russians into Japan. Perhaps there's intelligence from some unexpected source (Venona? Hey, that would be cool ...) showing Russian preparations and a real possibility of a Russian force landing in weakly-defended Hokkaido.
MacArthur is watching the invasion force approach the landing beaches from Bataan, his personal B-17. This is rammed by a suicide aircraft, killing all on board. Nimitz, as next senior commander in the Pacific, takes over but it really is too late to call off the invasion -- the political consequences of turning around when the invasion fleet has already been sighted by the Japanese are unthinkable. It takes some time for Nimitz to get a grip on the situation, which is rapidly deteriorating since the defenses are just as awful as Intelligence predicted (which is much worse than MacArthur allowed himself or his planners to believe.) A big scandal is already brewing over MacArthur's grossly optimistic preinvasion estimates...
Heck, I'm not going to write this whole thing for you. You take it from there. (Will be happy to provide my technical $0.02.)