Why were the Japanese so scared when the Soviets began invading then? As far as I am aware, a huge factor in making the Japanese surrender was the invasion of Manchuria. Which seems to indicate the Japanese were scared of the Soviets.
Seems strange for them to fear the Soviets so much if they actually had no capacity to invade Japan.
Japan's last hope for surviving the war would have been to exploit a falling out amongst the Allies over Europe and get the Soviets on their side to resist an American invasion. When the Soviets abrogated their nonaggression pact with the Japanese and invaded Manchuria that was the last nail in the coffin and the choice became either unconditional surrender or national suicide by resisting and suffering invasion and nuclear holocaust.
A combination of things. The USSR invading Manchuria basically destroyed their last card to play, the Kwangtung Army in Manchuria and meant that their plan, to use the Soviets as an intermediary to make peace with the WAllies through after they bled them out during Downfall. The USSR entering the war meant their plans where all for naught
Not to mention the Soviets entered the war after the IJN and their airforces were rendered impotent save for one last Kamikaze surge in the south, the Soviets could have probably invaded Hokkaido and Northern Honshu under those conditions with US help, and they really did not want the Soviets getting a piece of the home islands
Furthermore, for Japan the whole point of the war was to control China. Everything else was secondary and was done to that end, including the Pacific War against the USA.
The Japanese would be afraid of the Soviets because they were the only army that could directly take them off of the Asian mainland. Plus they'd loose Korea, which was part of their country.
The speed with which the Soviets overran Manchuria basically confirmed their deepest despairs, and so surrender became necessary.