This is blatantly false. Even after two atomic bombs had fallen on Japan, their military leaders -- and indeed most of their population -- remained committed to fight on until the nation of Japan ceased to exist. It was only the intervention of the emperor himself (who never gets involved in matters of war) that convinced the military and the people to surrender after the bombs were dropped.
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Denying Korea and Taiwan is easier said than done. Remember that Japan essentially won the continental war against China. It's best equipped and most experienced troops are there. Japan is also in an optimal position to control ocean access to Korea. Any boats have to travel through the East China Sea, or sea of Japan to reach Korea. Both of which are in range of the thousands of kamikaze planes built for the purpose of destroying vessels that sail close to Japan.
At this point, you're talking about conducting an invasion of a landmass even larger than the Japanese home islands, against better troops, and with more difficult naval access. It would undoubtedly incur substantially more losses than the 100 to 200 thousand inflicted by the atomic bombings. And to what end? A hostile military government would still be in power back in Japan.
Before these bombs were dropped, tens of thousands were dying on days with no large military operations.
The counter argument for that is it took two atomic bombs for Japan to concede. And after the 2nd one it was 3 days before they gave up.
Could a blockade have worked? Maybe. But how long would it have taken? Would we all just sat and watched while hundreds of thousands of Japanese civilians starved to death over a few years?
I hope I wouldn't have dropped the bomb on a populated city if I were in Truman's shoes, but without the benefit of hindsight, I'm not positive I would have decided differently.
On a side note, Japan is a wonderful country, particularly the countryside. My wife lived there for 13 months while we were dating, and I have many fond memories of trips around Japan with her. Lost wallets more common than not are returned to the police with money still inside. One of my colleagues once saw a man in a business suit sleeping on a park bench one morning, obviously drunk, and someone had placed the man's dropped phone and wallet on his chest.
It was already defeated, and offering to surrender, before the USSR joined the fight.
Totally surrounded/blockaded, no allies left in the world, bombed at will, what else could it do but accept defeat?