What caused the Japanese surrender in WW2, the bomb, or the Soviets?

Today is the anniversary of the day the second atomic bomb was dropped on Nagasaki.  On that same day, August 9, 1945, the Soviets began their invasion of Manchuria.  The Soviet invasion was a remarkable success, and by August 15th, the Soviets conquered all of Manchuria, captured over half a million Japanese troops, and reached the border of Korea.  August 15 would end up being the day Japan surrendered.  While Americans have always assumed it was the dropping of the two atomic bombs that caused Japan's surrender, the Soviet role in the defeat has started to get more attention recently.  In message board discussions on the issue, many people claim it was the Soviet entry into the war that convinced the Japanese it was time to surrender, and the bombs had very little effect.

While the American-centric view of the war has caused the Soviet's invasion of Manchuria to be almost completely forgotten, I think the belief they deserve primary credit for forcing Japan to surrender is a case of the pendulum swinging too far in the other direction.   It also seems that people are trying to transpose what happened with Germany to the war with Japan.  In the last days of the European war, the Germans fought furiously to avoid being captured by the Soviets, fearing what they would do to them.  I'm not sure the Japanese would have had the same fear of the Soviets.  The Germans were right to fear Soviet brutality, but much of that brutality stemmed from the Soviets seeking revenge against the Germans for atrocities they committed during their invasion of the Soviet Union.  I'm not sure the Japanese had the same reason to fear the Soviets as the Germans did. 

Another factor to take into account is, "Did the Japanese believe the Soviets were truly a threat to their home islands?"  An amphibious invasion of the Japanese home islands would have been an enormous undertaking, and I haven't seen anything to indicate the Soviets had the equipment to do it.  The American forces had all the landing craft that had been used previously in the Pacific, all the landing craft that had been shipped over from Europe, and from I've read they believed they just barely had enough to invade Kyushu, the first home island they were targeting.

Of course, the Japanese could have been fearful the Soviets would have participated in the American invasion, or that they would get a zone of occupation in Japan if the war lasted longer.  Once again, though, the question must be asked how much would they have had to fear Soviet atrocities in these situations.

In the end, I think Soviet entry into the war must be given credit for the Japanese surrender, along with the atomic bombs being dropped.  The loss of Manchuria deprived Japan of much needed resources, in addition to the capture of the half a million troops.  Before the Soviet invasion, the Japanese might have believed they would be able to negotiate a peace that allowed them to hold on to some of their conquests on mainland Asia, but the loss of Manchuria, and the potential for the Soviets to conquer more of their territories, convinced Japan this was never going to happen.

One good thing that has come about from the debate is that a previously forgotten moment of history is being re-discovered.  There is very little in the way of books or documentaries examining how the Soviet conquest of Manchuria was carried out.  I've embedded a doc I found outlining the Soviet campaign.  It's pretty decent, although it insinuates the timing of the invasion was arranged to take place on the same day as the Nagasaki bombing.  In reality, the timing is just a coincidence.  The date for the Soviets declaring war on Japan had been decided at the Yalta conference, when Stalin promised to go to war against Japan three months after the war against Germany ended.  The European war ended on May 8, meaning the August 9 start date for the Manchurian invasion was set from that.