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...without a mainland troop invasion by US troops...
I don't know why, every time this endless argument comes up--atom bomb versus invasion--nobody mentions the perfectly viable third option: blockade.
Japan's home islands produced almost nothing, in terms of natural resources. No oil or coal for fuel. No ores for metal and manufacturing (which is why they were importing scrap metal from the U. S. even as Pearl Harbor was in process). Very little food, other than rice and what fish they could catch. No rubber.
So ring the islands with an iron curtain of U. S. warships. We could have set up a blockade so tight that not a trawler could have penetrated it. Yes, the few Japanese aircraft remaining would attack the ships, but soon they'd be gone. Japan's sub fleet would try the same, with the same result. It might have taken until 1947, say, but eventually the Japanese people would have gotten tired enough of eating paper and wood that they'd have done something about the militarists who controlled the government (who themselves were probably pretty tired of eating paper and wood). There would be no gasoline, no new production of weapons, no ordnance produced, nothing to eat.
The downside for some might have been that even more Japanese would have died of starvation than died of atomic weapons, but very few Americans would have died in a blockade.
It's exactly what the Germans planned to do to the Brits, after their attempt at invasion was ended by the Battle of Britain. They then hoped to cut the British Isles off from all imports of food, fuel and military goods with their huge U-boat fleet--assuming they won the Battle of the Atlantic. As it happened, they lost that battle. But the U. S. definitely wouldn't have lost a Battle of the Blockade of Japan.