[PeterPrinciple]

Was it the atom bombs, or the Soviet declaration of war,

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Like most things in life it is never one thing but several converging. That said, it appears they were aware of the Soviet possibility and willing to deal with it, but the Atom bomb WAS A COMPLETE SURPRISE AND UNKNOWN FACTOR making the decision more urgent.

[alfa6]

The Bomb was pretty much the factor that pushed the Emperor to call it quits. The second Bomb convinced the Emperor that the US had more Bombs and would not hesitate to use them on Japan.

The Japanese Military could always hold out hope that the Japanese Spirit would prevail against the conventional arms.
That hadn’t worked out to well for the Japanese so far, eh?

However when your enemy can send over 1 aircraft that wipes out a fair sized city and inflicts horrible death and injury to your citizens it’s time to toss in the towel.

Had there not been the second Bomb the Japanese would have fought on a while longer but IMHO would have called it quits by the end of August or so. Logic behind the previous statement by the end of August the full extent of the Soviet advance would have been known and made clear to the Japanese that there was no hope left. Maybe, I hope :-)

Regards

alfa6 ;>}