Such as? Under the circumstances, the use of two atomic bombs was morally sound. The alternative (your alternative) is stalemate and a militarized Japan that remains a threat until they develop nuclear weapons. Then, if we are lucky, a Cold War in which we hope Japan does not ally with the Soviets.
Your other question is what are the limits to war? There are none and destruction of the world is possible. But it would not be the USA destroying the world.
How about indiscriminate destruction of civilian populations in massive bombing campaigns?
Under the circumstances, the use of two atomic bombs was morally sound. The alternative (your alternative) is stalemate and a militarized Japan that remains a threat until they develop nuclear weapons. Then, if we are lucky, a Cold War in which we hope Japan does not ally with the Soviets.
First of all, you take a major event that played out over the course of years and present it as if history had reached a point where there were only two possible scenarios. That's a ludicrous approach that treats every human interaction (or interaction between nations) as nothing more than a coin flip.
Secondly, how could there possibly be a diplomatic or moral objection to a strong relationship between Japan and the Soviet Union at that time? The U.S. had no qualms about maintaining friendly relations with the Soviets in the 1940s -- even to the point of selling out Eastern Europe to the Soviet Union at Yalta in 1944?