I REALLY don't want to go into this, but the Japanese weren't defeated by nuclear weapons (although undoubtedly eventually they would have been), by that point of the war they were beaten plain and simply by conventional means. By that point in the war, they had no navy left, no merchantmen, no oil, no industry etc. It was a done deal.
And perhaps more importantly, the Soviet Union declared war on Japan too and they have completely swept across China and Korea. If there was one thing the Japanese were more scared of than nuclear weapons was the Soviets. They understood that a Soviet occupation would have meant the utter obliteration of everything they thought that made them "Japanese". Their Emperor, their culture, religions, traditions, class system etc.