They were trying to get the Soviets to broker a peace for them, but then the Soviets declared war on them so that didn't work out. The only thing left was to get the top brass to realize that the only thing left that they could do was talk to the Americans, but going through the cables between them it doesn't seem that the nukes factored into that discussion at all - they just saw it as yet two more bombings in a whole host of bombings they had suffered at that point.