> Actually, Japan were ready to throw in the towel as a result of the tripartite pact being enacted, and Russia joining the fray.
People say this a lot, but no one has clearly explained how the U.S.S.R. would have been able to stage an invasion of the Home Islands of Japan. It's not a matter of simply ferrying a bunch of people over the Trans-Siberian railroad, you also need a navy, sufficient air cover (which if I remember my geography correctly would have almost necessarily meant carrier-borne aircraft), and the logistics to back that all up. Japan knew this just as much then as we realize it now.
The Soviet gains after their entry into the war were in China and Korea, not in Japan itself. Their entrance into the war was earlier than expected on the Japanese's part, but they knew the U.S.S.R. would declare war (seriously, you may consider their senior and military leadership stupid, but that stupid?).
Likewise the attack on the Kwantung Army in China was surprising in that it came from every direction possible instead of the one direction they expected it from. But they knew the Soviets would attack, and in fact the attack fell while the Kwantung Army was changing positions to better guard the direction they believed the attack would actually come from.
So certainly the Soviet attack came as a shock, but it was militarily no more severe than any other setback to the Allies.
> but outside of the US the majority agree that the USSR caused the surrender.
Don't take this the wrong way, but what does an opinion poll have to do with whether some event actually happened in a given way or not? It either did or did not happen for the causes described. If the rest of the world has better evidence for their belief then point to the evidence, but don't point to an opinion poll as prima facie evidence of something.
In the event we have evidence on the Japanese side that the atomic bomb proved a useful "excuse" to permit the Emperor to surrender. The Japanese were clearly ready to fight to the death (just witness the Battle of Okinawa), and that applied just as much against the Red Army as it did against the Americans. So simply changing the uniform of the person holding the rifle wouldn't have significantly affected the strategic calculus when the Emperor had to explain his decision to his people.
On the other hand, the atomic bomb was this technological doomsday device that even the most hardened Japanese militant couldn't speak against. You couldn't fight it in the battlefield; you couldn't simply hunker down like a brave citizen and start picking up pieces after the bombs had been dropped, now there were no pieces left to pick up at all.
Japan had nothing like it, and as such it gave the Emperor a clear line of reasoning for why he had to (had to) surrender for the sake of his people. It was something completely different and from out of this world, as opposed to a simple Soviet declaration of war.