Growing up in North America, we are often told that the dropping of the atomic bombs were used as an alternative to a land invasion. We are also told that the bombs succeeded in forcing the Japanese to surrender. However the official surrender was signed almost 3 weeks after the bombs, meanwhile the Soviets had already taken most of Manchuria and were planning a land invasion of mainland Japan along with the Korean Peninsula.
So basically, I'm wondering if someone could walk me through what the Japanese military were thinking and whether they weighed the American threat or the Soviet threat with more or less importance.
The Japanese strategy towards the end of WW2 boiled down to "win a battle, then sue for peace." They wanted to convince the US that a protracted war and invasion would cost a tremendous number of American lives and that success wasn't guaranteed, in hopes that the US could be induced to be brought to the negotiating table and would accept something short of complete surrender. (The military leadership wanted to hold out for no occupation and self-disarmament - almost certainly in vain, considering the German resurgence in the interwar period.) They believed that asking for a negotiated peace after an unbroken string of Allied victories would simply result in a rebuff, and would be disastrous for their own morale. Thus, they needed a solid victory or at least a bloody repulse of the Americans before they attempted to negotiate.
In fact the Japanese had correctly anticipated that the US would attempt to invade Kyushu, and had concentrated a large part of their forces there. If they could throw back the US invasion, or at least maul it severely, that would provide the opening they were waiting for.
Part and parcel of this strategy was the conception that the Soviets would play a key role in the negotiations. The Japanese didn't actually want to come under the Soviet umbrella, as it were - but saw that the implied threat of doing so could be used against the US. Presumably the US could have been induced to accept peace on more lenient terms if the alternative was a Japan which opened itself to the Reds.
So the events of August 6-9 took the two legs of this strategy and kicked out -both of them at once-. The US, armed with atomic weaponry, did not actually need to invade in order to win (and the Japanese, after initially assuming that the US could have only created a single bomb and then having been demonstrated to be incorrect, then thought the US had significantly more of them available). They thought that the US would not even bother to invade if it had the capacity to completely annihilate Japan without doing so. Conversely, with the Soviets in the war, they were hardly likely to shield the Japanese from the Americans. Thus, the prospect of achieving the necessary victory-or-at-least-bloody-stalemate had receded to nothing and the diplomatic situation the Japanese leadership was counting on (completely in vain, but they didn't know at the time) had proven to be impossible as well.
A lot of the criticism of the orthodox history is phrased as "weren't the Japanese more worried about the Soviets than the nukes?" In that respect, not so much - while the Soviets certainly romped through the Japanese troops in Korea and Manchuria, they had little in the way of capacity to invade the home islands and Japan had already resigned itself to losing most (if not all) of its overseas holdings as a result of the war. But the Soviet invasion represented a collapse in the hopes for a pro-Japanese Soviet intervention, which put a wrench into the gears of the Japanese war plans (such as they were.) That, on top of the atomic bombs...
If you're interested in further details, I highly recommend Richard Frank's Downfall, which has plenty of details on both the military and diplomatic situations.
Hi there -- while you wait for more answers here, you may also be interested in this section of the FAQ on the atomic bombs and their effects. The argument over what really made Japan surrender is an interesting one because given the available evidence, one can make a case for a variety of factors being important. Happy reading!