Thing is, we simply don&#x27;t know how big a reason it was.<p>Certainly it was a very conveniently timed shock, but unlike the bomb it wasn&#x27;t a reason the Emperor cited for surrendering.  And the threat the USSR represented to the home islands, especially Honshu, was limited, although I&#x27;ll bet the way the Red Army tore into the IJA in Manchuria was convenient in helping to cow its hotheads.<p>I think the most we can say it that it might have been necessary, but it probably wasn&#x27;t sufficient.