It cannot be definitely said that one or the other was the cause.

Japan's situation in August 1945 was: loss of all outlying territories and defeat in every theatre (except mainland China). Very limited resources due to blockade. Cities being wiped out and precious little the air forces could do about it.

But, Japanese codes of honour and the dysfunctional way politics was run- with meaningful and honest discussion of realities almost impossible- meant the actual process of surrender was almost impossible to confront. Militarists controlled the government and they simply wouldn't be responsible for losing.

With the situation desperate, a small clique set about trying to find Japan a way out of the war. The plan was, basically, to approach the USSR and have them mediate with the US and British. Most of the Japanese leadership basically thought that they were in a position to negotiate, and not just on the preservation of the Imperial family.

Hiroshima was bombed, but this barely dented the Japanese position. Various leaders either denied an atomic bomb had been used, or convinced themselves that the US could not have many.

With the hopes of the small, clandestine peace group pinned on Soviet co-operation, it was a shock when the delegation in Moscow was told that in fact the Soviet Union was declaring war on Japan. with the invasion of Manchuria already started. At the same time, the second bomb was dropped on Nagasaki. This shifted the position of the Prime Minister and Foreign Minister (though not of the Army leadership itself).

There followed two days of Council discussions with members still basically in denial about their negotiating ability. Eventually, the impasse was put to the Emperor who, in his roundabout way, made it known that he expected the war to be ended. In his speech, he made references to the atomic bombings and the lack of defence against an invading force.

Those who regard the atomic bombings as a war crime, or as mortally wrong, have a stake in 'proving' that the Soviet invasion was the prime, or only, instigator of the Japanese surrender. While it was a factor, and while it's true that in the West we have historically tended to focus entirely on the A-bombs, the contention is false.

Hi there -- this is a fairly well debated piece of the historiography of WWII, so you may not get one single answer on it. A good place to start while you wait for more answers here is this section of our FAQ.