I do believe the bombings were a strategic show-of-force to the Soviet Union since they were coming out of the war as a global power; USSR were instrumental in ending the war.
An interesting perspective summarized here [0] is that the bombings were not just unnecessary to end the war and were conducted as a show of force to the Soviets. They were conducted in an attempt to end the war ASAP, before a planned Soviet ground invasion of Japan so that the US wouldn't have to share control over post-war Japan with the Soviets.
The fact that the military is still awarding Purple Hearts that were manufactured for the invasion of Japan tells me that it was more than justified. Japanese intelligence itself estimated 20 million casualties during an invasion.
We also have Germany as a point of comparison. 1945 Was the bloodiest year of the European theater, a fact made even more palpable considering that the European war ended in May. Compound this with the fact that the ratio of troops killed vs surrendered for Germany was about 4:1 versus 120:1 for Japan (granted this isn't really a straightforward comparison, Japan was often fighting over heavily fortified islands that afforded little opportunity to encircle large formations which is what precipitates most surrenders).
> [Japan was] offering to surrender, before the USSR joined the fight.
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.
Offering to surrender conditionally. Specifically, on the condition that it keep Korea and Taiwan (and maybe Manchuria, too), and that it's military government remain largely unchanged. This is was not a surrender that was acceptable to the Allies.
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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.
Not invading or dropping nukes would mean the deaths of millions. The Japanese would still fight, Japanese and Allied planes would still get shot down and warships sunk. Japanese Civilians would still be dying at a high rate, from bombings, starvation, lack of medical supplies, etc.
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?
Not only did it take two atomic bombs, but there was still an attempted coup to continue the war after the second bomb was drooped. [0] On top of that, the tendency for Japanese troops to feign surrender as cover for suicide attacks, and the particulars of the attack on Pearl Harbour, plus some racism all added to a combination of justified and unjustified mistrust of the Japanese military and the chances they would adhere to terms of surrender without having an occupation force in the home islands.
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.
Not mentioned on that list: It made the sort of stories that circulated in post-WWI Germany, about the old men selling out while they could still have kept fighting, much less plausible.
If the war aim was the permanent political defeat of Japanese fascism, then this was important. You don't just want the leader's signature on paper, you want defeat to be undeniable to everyone.
In both cases, the naval blockade left a lot of civilians hungry, and was great military value-for-money. But politically, that was easier to forget afterwards. Your adversary's ability to destroy whole cities with one plane was not.
(In germany's case, having two different armies in Berlin for half a century was also kind-of hard to forget.)
I don't think the war aim was the defeat of japanese fascism - perhaps, japanese militarism. After all, the basic element of japanese fascism (the Zaibatsu) essentially persists today. It's also notable that japan, while technically a democracy, has had the same party in power since 1955.
This always seemed like an odd conspiracy theory to me. It supposes that the U.S. is one the one hand so cavalier about dropping atom bombs that they're willing to destroy Japanese cities merely as a show of force to a potential rival, but on the other hand that they're so reserved about using it that they wouldn't use it to gain victory against the country they're engaged in a destructive war with.
For example, the fire bombing of Tokyo killed more in a single day than either Hiroshima or Nagasaki (though radiation sickness would eventually bring the Hiroshima total to slightly larger).
The only real difference with the atomic bombings was it was one bomb from one plane.
I would recommend reading into the firebombing campaing against the Japanese civilian population.
It was specifically aimed at maximizing civilian casualties, the same as burning of Dresden.
Something which would be interpeted as a war crime and terrorism today, wholly sanctioned by US military high command in the philippines at the time.
At the time I don't think there were any serious discussions about whether the bombs where "necessary". They where a powerful new weapon, and the US was in the middle of a war. Of course they were going to use it, it is unimaginable that thew would decide not to use a powerful weapon which was available to them.
Possibly, historically speaking, atomic bombs had to be used - on cities - at least once for humanity to realise the full horror of these weapons. Maybe no-one would have believed the destruction had they not been used at that time, and instead a far more terrible demonstration (with multiple actors?) would have subsequently become inevitable.
As a tangent to your comment, it always good to remember that big geopolitical decisions are never made in a vacuum, so when someone says “country X did Y because of Z” it usually misleading.
Some of the most fascinating reading I’ve done is the Pentagon papers because they go into exquisite detail about all of the thinking that resulted in the US’s involvement in Vietnam. It wasn’t just the Domino Theory, it wasn’t just containment of China, it wasn’t just getting France’s support for NATO, it wasn’t just Truman’s, JFK’s and Johnson’s efforts to look tough on communism.
I also recommend The Shadow Peace, a short 14-minute video analysis on the utility of nuclear weapons and their role in our modern era of relative peace: http://www.fallen.io/shadow-peace/1/ (unfortunately no HTTPS)
It's a slightly more optimistic take on the tragedies of the 20th century.
I do believe the bombings were a strategic show-of-force to the Soviet Union since they were coming out of the war as a global power; USSR were instrumental in ending the war.