The "don't understand psychology" line seems needlessly insulting.
Also, the Soviet entry into the war was far more than "just another military defeat". It was the entry into the war of yet another hugely powerful enemy, in a nation that was already seriously over-matched by its enemies. It represented the loss of the delusional, but widespread, hope that the Soviet Union would help negotiate an end to the war. Finally, it helped shatter the last thin hope of the hardliners in the Militarists. They had been arguing that one final battle would allow Militarist Japan to gain terms by making the conquest of Japan too bloody for the Allies to continue. The very quick and decisive victories of the Red Army showed how woefully outmatched Japanese armies were, and called into serious doubt the hopes of inflicting huge casualties on the enemy on the battlefield.
I personally might even agree that the nuclear bombings had more of an impact on the decision to surrender than did the Soviet entry into the war, but looking at all the facts, I don't see how anyone could argue that it was wholly one or wholly the other. The Soviet entry into the war was a huge blow to the pro-war faction, regardless of whether or not the nuclear bombings was an even greater blow.
There were many hardliners within the Militarists who wanted to continue fighting, right up until the bitter end. But it is interesting that even Anami argued not that Japan should go down fighting to preserve its honor, but because there was still hope of inflicting such severe losses on the enemy that terms other than unconditional surrender are granted. As far as I can tell, the hardliners in Germany did not even argue that, just that Germany should keep fighting until the bitter end simply to avoid a repeat of the "dishonor" of 1918.
It's not an insult not to understand psychology, or any subject.
I can't find one great power that surrendered because a powerful enemy entered the war. It might be part Soviet attack part nuke, but it's definitely not 60 40 but 99-1 for the nuke. When Prussia, Sweden, France (many times), Poland, Germany, even Turkey were attacked from all sides by powerful enemies, they fought on because people delude themselves into thinking they can win. Some of them, like the French up to a point, the Venetians and the Prussians, actually did. All it takes is a charismatic leader and a propaganda machine to convince people to keep fighting. Even in 1943, most Germans thought they could beat the world.
Much like the strategic bombing campaign was. Yet, when Tokyo went up in smokes in March 1945, Japan did not surrender under that pretext. Invasion may not be as psychologically devastating as a nuclear bomb to the civilian population; but it would have been, perhaps even more so, to the Japanese command who could actually make the decision to continue or end the war.
The militarist and the peace advocates were both banking on Soviet neutrality. The militarist, so that the Kwantung Army could be used to fend off a US invasion of the homeland; and the peace advocates, as a neutral mediator to allow for a better peace agreement.
The Soviet declaration of war and its annihilation of the Kwantung Army shattered both these hopes. Additionally, there was a real fear of an invasion by the Red Army against the homelands of Japan as there was a fear of an invasion from the US Army. A major difference between these two armies however is that the Red Army was willing to fight to the last man, much like the Japanese. The US and the Western Allies on the other hand much more valued the lives of their soldiers. Thus, when the SU joined the war, it was clear to Japan that there would be no negotiated peace. It was surrender or annihilation, both by the atomic bombs and Russian bodies.
Whilst I do agree that both the nuclear bomb and the Soviet DoW were important to the surrender of Japan, I'd personally argue that the Soviet DoW was much more influential.
Good point but the bombing was expected, a well drilled propaganda routine, and there were still cities standing. Life in Japan sucked, but life in Sweden during the Great Northern War or Germany in the first world war sucked too. It takes many years for people to crumble from sheer attrition. I don't think the Japanese feared invasion as much as their elite was excited about it. They deluded themselves into divine winds and samurai myths, and to give them credit, it would have been their best battle sequence since the early string of victories in 41. Japan predicted the allied plan 100%, which isn't an accomplishment since there was only one way they could go, but it would have been bloody.
As for the bomb, it was a shock, unexpected, seemingly godly power, and a complete jaw dropping moral blow.
For the mentality of the USSR, people in war don't think that deep that the Soviets were okay with losing men and the Western allies weren't. There were legitimate Japanese hopes of winning a negotiated peace with the US, but I don't think they all believed the Soviets were tough and hardcore to where if the US got tired the Soviets would still think it was worth it to conquer Japan. Why? To make it communist? If Stalin was willing to trade lives for more land, he would have had Churchill and Roosevelt go with the British plan to invade Yugoslavia: today, most scholars think he would have made it to France.
Even if the Soviets were tough, a Japanese officer part of the fight to the last man faction is thinking "this is our home, these are our beaches, and we will kill them all". Russians were tough when they invaded Prussia, but the Prussians (ironically with good results) deluded themselves into the same thinking. Now if a magic hurricane cast by the Tsarina swept away Berlin? That would convince them to surrender.