Which caused Japan's surrender: the atomic bombings or the USSR declaring war?

Which event most influenced Imperial Japan to surrender, the atomic bombings or the Soviet declaration of war?

What would have happened if one event happened without the other?
 
I'm 90% sure it was the Russians. The Japanese had always planned to have Russia as a semi-friendly neutral through which they could conduct diplomacy to get a peace (on their terms). Operation August Storm basically told them that everyone in the world now hated them and their time was up.

The nukes I think were seen as simply a big nuisance, at least by the administration. The Japanese after all had already gone to the extent of crashing planes with people in them into boats and whatnot, they were already ruined. Besides, after about five nukes were dropped, all they could do after that is stir up some rubble (and radiation poisoning, but I don't think that was on their minds at the time).

To the Japanese, the nukes made things harder, the Russians made things impossible.

In answer to your second question:
Nukes without Russia: nukes stir some rubble around, Operation Downfall happens eventually, lots of people die and the war is over by 1946.
Russia without Nukes: A few assassinations happen in Tokyo (the peace party or whatever they got called had the nukes as a good reason to end the fighting, Russia was the prompt that made them try to end it). If the peace party lives, the war ends much the same as OTL. If the war party has control, probably revert to Downfall and people dying.

- BNC
 
I'm 90% sure it was the Russians. The Japanese had always planned to have Russia as a semi-friendly neutral through which they could conduct diplomacy to get a peace (on their terms). Operation August Storm basically told them that everyone in the world now hated them and their time was up.

The nukes I think were seen as simply a big nuisance, at least by the administration. The Japanese after all had already gone to the extent of crashing planes with people in them into boats and whatnot, they were already ruined. Besides, after about five nukes were dropped, all they could do after that is stir up some rubble (and radiation poisoning, but I don't think that was on their minds at the time).

To the Japanese, the nukes made things harder, the Russians made things impossible.

In answer to your second question:
Nukes without Russia: nukes stir some rubble around, Operation Downfall happens eventually, lots of people die and the war is over by 1946.
Russia without Nukes: A few assassinations happen in Tokyo (the peace party or whatever they got called had the nukes as a good reason to end the fighting, Russia was the prompt that made them try to end it). If the peace party lives, the war ends much the same as OTL. If the war party has control, probably revert to Downfall and people dying.

- BNC

Nukes without Russia: downfall abandoned or redirected since invasion sites have 1:1 ratio of defenders. Instead, operation starvation continues, famine leads to Japanese collapse by 1946
 
Both, 70% the russian, 30% the bomb, as much the japanese were willingly to commit national suicide...those bombs would have worked
 
But now the war has lasted for nearly four years. Despite the best that has been done by everyone – the gallant fighting of the military and naval forces, the diligence and assiduity of our servants of the state, and the devoted service of our one hundred million people – the war situation has developed not necessarily to Japan's advantage, while the general trends of the world have all turned against her interest.


Moreover, the enemy has begun to employ a new and most cruel bomb, the power of which to do damage is, indeed, incalculable, taking the toll of many innocent lives. Should we continue to fight, not only would it result in an ultimate collapse and obliteration of the Japanese nation, but also it would lead to the total extinction of human civilization.


Such being the case, how are we to save the millions of our subjects, or to atone ourselves before the hallowed spirits of our imperial ancestors? This is the reason why we have ordered the acceptance of the provisions of the joint declaration of the powers.


Answer's right in Hirohitos' message.

No mention of Russia, other than reading them into

the war situation has developed not necessarily to Japan's advantage

For a good dose of understatement
 
I think it was 70% bombs (more direct) and 30% Russians. With nukes and without Russians the Japanese would relocate some of their stuff to Manchuria/Korea but the home islands would probably capitulate after a few more strikes at most. The IJA on the mainland would be uprooted over the next year.
 
I think it was 70% bombs (more direct) and 30% Russians. With nukes and without Russians the Japanese would relocate some of their stuff to Manchuria/Korea but the home islands would probably capitulate after a few more strikes at most. The IJA on the mainland would be uprooted over the next year.

There was no relocating, as by August, the IJN and the merchant fleet was gone or hiding in port, Sea of Japan was an American lake, thanks to the subs.
 
Both. We have to remember that even after all of that happened the Japanese cabinet was still equally divided on whether or not to surrender (the Emperor cast the deciding vote), and there was a coup attempt to stop the surrender. That said, fear of falling to Communism was a greater motivator than the bomb.
 
I don't think you can fully grasp the mentality of the Japanese leadership without taking in both factors at once. Their world is coming down around their heads. They had hoped there might be some chance that as long as the Soviets weren't in combat yet, they could persuade the Soviets to be an honest broker for a conditional truce, but now the Soviets are invading, and they're steamrolling over Manchukuo, so all hope is lost there. How long before they invade the home islands? Meanwhile word has reached them that the Americans are using a new super-weapon against Japanese cities. Yes, the biggest cities are already being fire-bombed, but how many of these new weapons does America have?

The emperor and others have already realized that the war is a lost cause and this combination collapses their will to fight on. Might as well take their chances with peace. And probably better to surrender to the Americans whole than to wait until the Soviets arrive and get split in half, or worse yet, given over to the Soviets.

And even then, as has been pointed out, the cabinet was divided and the military wasn't fully prepared to give up.
 
Trusting a governments pronouncements, public pronouncements as a source on their motivations is poor sourcing. Try searching google scholar with "review article" as a search term. Review articles are a us academic convention in historiography where a scholar is asked to sum up debate as it exists.
 
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