Historian posits that it was the Soviet attack that caused Japan to surrender

[Larry1]

Historians have long debated what caused the Japanese to surrender in 1945. The most common view was that the two atomic bombs caused the surrender. A historian from University of California at Santa Barbara posits that the real cause was the entry of the Soviet Union into the war with Japan. 

I will review briefly the sequence of events leading up to the surrender. By the Summer of 1945 Japanese sea and air forces had been considerably degraded. The US was close enough to the Japanese home islands to conduct a formidable bombing campaign. Japanese industry was nearly destroyed. Japan was largely cut off from sources of most raw materials.

In early August the US dropped the first atomic bomb on the city of Hiroshima. But still Japan was not seriously considering surrender. Within days, in accordance with Stalin's promise at Yalta (it might have been first made at Teheran), the Soviet Army attacked Japanese forces in Manchuria and had considerable success against them. Some days later the US dropped  the second atomic bomb, on the city of Nagasaki. Within days the divided Japanese Supreme War Council decided to surrender.

Tsuyoshi Hasegawa - a highly respected historian at the University of California, Santa Barbara - has marshaled compelling evidence that it was the Soviet entry into the Pacific conflict, not Hiroshima and Nagasaki, that forced Japan’s surrender...

http://www.boston.com/bostonglobe/ideas/articles/2011/08/07/why_did_japan_surrender/?page=1

If you are interested in this I suggest that you read the article found at the link. It contains a discussion of the professor's research, along with some criticism of his conclusion.

I don't take a position either way because I haven't read Professor Hasegawa's analysis of the documentation.

[jone]

Larry,

I don't doubt that this professor was right. 

Japan was looking at any possible post war for themselves and would have realized that they would get better terms from the United States than from Russia.   The prospect of Russia seizing outlying islands or of participating in an occupation of Japan was enough to sway the government.

[mendeleyev]

Aww shucks....Russia wouldn't try to take any of their islands, right?   

[jone]

Aww shucks....Russia wouldn't try to take any of their islands, right?   

Russia believes in colonial expansion.  Without the colonies.

[Muzh]

Russia believes in colonial expansion.  Without the colonies.

Ahem, look at Puerto Rico.

[jone]

Ahem,  you can have it back!

Now if you would have said Hawaii, I might have said that, yes, we took that Pacific pearl.  But the only thing that happens to those who go to Puerto Rico is that they come back eaten up by the noseeums.  I'm convinced that Spain is enacting revenge on those Yankee invaders by having all of those little creatures eating up the non-natives.

[Brasscasing]

Historians have long debated what caused the Japanese to surrender in 1945. The most common view was that the two atomic bombs caused the surrender. A historian from University of California at Santa Barbara posits that the real cause was the entry of the Soviet Union into the war with Japan. 

What the good professor fails to mention is that it was the Japanese/Soviet neutrality pack of '41 (a la Molotov–Ribbentrop Pact) that allowed the Japanese to wage it's war in the pacific in the first place...

Soviet–Japanese Neutrality Pact

..."The Soviet–Japanese Neutrality Pact (日ソ中立条約 Nisso Chūritsu Jōyaku?), also known as the Japanese–Soviet Non-aggression Pact (日ソ不可侵条約 Nisso Fukashin Jōyaku?) was a pact between the Soviet Union and the Empire of Japan signed on April 13, 1941, two years after the brief Soviet–Japanese Border War (1939). The pact was signed to ensure the neutrality between the Soviet Union and the Empire of Japan during World War II, in which both countries participated."...

http://en.wikipedia.org/wiki/Soviet%E2%80%93Japanese_Neutrality_Pact

So, during the entire four years of the Pacific war the USSR managed to keep their back door safe and avoided becoming involved in a two front war with the empire of Japan. A war the rest of the Allies were desperately fighting.

Now, sometime around Jul 45 the Japanese, based on the fact they did have a non aggression pact with the USSR, had already started petitioning the Soviets to intervene on their behalf for more favorable surrender terms (keep in mind the Allies wanted unconditional surrender)...

Surrender of Japan

..."Together with the United Kingdom and China, the United States called for the unconditional surrender of the Japanese armed forces in the Potsdam Declaration on July 26, 1945—the alternative being "prompt and utter destruction". While publicly stating their intent to fight on to the bitter end, Japan's leaders (the Supreme Council for the Direction of the War, also known as the "Big Six") were privately making entreaties to the (technically, though temporarily) neutral Soviet Union to mediate peace on terms more favorable to the Japanese."...

http://en.wikipedia.org/wiki/Surrender_of_Japan

However, Stalin had already promised Roosevelt that the USSR would enter the war against Japan after the Germans were defeated...

Japan–Soviet Union relations

..."At Yalta in February 1945, Stalin promised Roosevelt that the USSR would enter the war against Japan 90 days after the defeat of Germany, which took place in May. It met that timetable by shifting large forces across Siberia. In April 1945, Moscow annulled the neutrality pact. The Soviet invasion of Manchuria began on August 8, 1945, after the atomic bombing of Hiroshima (August 6)."...

http://en.wikipedia.org/wiki/Japan%E2%80%93Soviet_Union_relations

Alright, that's the historical background setting up the Japanese surrender.

What forced/caused the Japanese to capitulate and announce surrender?

1) The Japanese recognized that a diplomatic options for more favorable surrender terms were gone now that the Soviets were no longer neutral but belligerents.

2) That they might be able to fend off one invasion coming from the South (the US) or one invasion coming from the Northwest (USSR) but not both.

3) The Soviets would be able to overrun the Japanese effectively taking the main island at about the same rate as the US creating a Germany like situation with their homeland being partitioned between the Allies.

4) Regardless of the Atomic bombs the US were bombing the Japanese uncontested on a daily basis by this point so whether Atomic or conventional, the damage/casualty rate was only going to increase.

5) The Japanese merchant and Navel fleets (the lifeblood of the empire) had ceased to exist.

It's a not accurate for the professor to suggest an alternate history by pointing to the USSR as the "real" reason for the Japanese surrender. It was a combination of several strategic/tactical factors certainly including the threat of further deployment of atomic bombs.

Besides, 'the Soviet factor' is not a revelation by any stretch of the imagination. Historians have always included the USSR entering into the fray as a factor in the Japanese surrender but certainly not the factor.

It should be noted that historically it's generally accepted that it was strategically advantageous for Stalin to declare war on Japan at that point as he was able to take advantage of the Japanese surrender to seize land, most notably occupying the Kuril Islands. If the USSR had not seen a strategic advantage at the time I doubt they would have honored Stalin's pledge to declare war in the first place.

Brass

[Slumba]

Ahem, look at Puerto Rico.

Seriously, tell everyone you know, if PR wants to break away from the USA, there are many that will support it. 

And take your $72 billion of debt with you 

[Muzh]

Seriously, tell everyone you know, if PR wants to break away from the USA, there are many that will support it. 

Are you serious? Congress will unleash the Army, a la Grenada, to stop the slave insurrection.

And take your $72 billion of debt with you 

The first issue on the agenda of the new republic would exactly that.