Japan didn't surrender because we dropped two nukes on them. They were willing to endure an invasion by the Americans. What got them to surrender was when the USSR declared war against Japan a day or two before. They didn't want a Soviet invasion because they new the outcome would be far worse.
With Afghanistan, and the Soviets found this out as well, is you can't bomb a country into submission if the majority of the population already lives in stone age conditions. That war was strickly to have control of the poppy fields.
You're misunderstanding conquest vs war. Bombing stone age people for conquest works poorly. If you just want to eliminate a threat (war) it works great. E.g. the Taliban cant export Jihadis if their cities are nuked, fields destroyed, and borders a 20 mile free-fire zone.
As for Japan, sure, the Soviet entry into the war precipitated surrender and the USA was planning an invasion, but that reflects overplanning more than necessity. Sitting back and bombing the home islands was 100% viable. An island nation bombed into the stone age is no longer a threat because they cant leave.
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