There were actually two factors that forced the Japanese to surrender, only one of which were the atomic bombs. The other factor was the Russian invasion of Manchuria. Many people feel that this was a sideswow, but to the Japanese, it was far more. In Manchuria, the Japanese had their largest army, and one of their largest sources of manpower and natural resources. Between the atomic bombs, the Russians invaded and defeated the Japanese Kwantung Army in about 5 days, and with that loss, the Japanese realized that they no longer had any hope of building any type of an industrial base that would allow them to continue the war. In addition, the Japannese government had been using the Soviet Embassy to send peace feelers to the Alles, and the Soviet invasion shut that door tighter than a coffin. So with the US superweapon, the Allies closing from the south and east, and an even larger army marching unopposed from the north and west, The Japanese War Cabinet resigned or committed suicide, paving the way for the new government to surrender.
I am of the opinion that it took both events to force the surrender. I remain unsure that the Japanese would have surrendered were it not for the Russian attack as well. Did it save lives? YES!!!! The Japanese had a very large army still available to them on the home islands. In addition, they had many fighter aircraft, but few trained pilots. In the end, an invasion would have been very costly, as the Japanese would have fought to the end. Estimates on US casualties of such a battle range from 50,000 (the-quick-invasion-followed-by-a-quick-surrender scenario) to well over a million. For Japanese civilians, the estimates were far higher. In the end, such a battle would have been well described by one line from the Lord of the Rings: If this is a victory, then our hands are not big enough to hold it.
Dean.