Zack
99% hot gas
But we already know how devastating nuclear weapons are. We don't need this hypothetical to be aware of that. I don't think not retaliating would say that humans can be restrained in their use, either. Rather, I think that the hypothetical enemy used them in the first place says that humans could be very wanton in their use of nuclear weapons, regardless of whether there was a retaliation.
I think in the scenario described, retaliation, at least of some sort, makes sense. This is assuming that it's certain who is launching them and that they're launching them intentionally. Not retaliating would leave the door open for the same nation to do the same thing to another nation in the future, and might also invite others to assume they wouldn't be mutually destroyed by a first strike. I can see the argument for a selective retaliation, primarily against leadership and military targets. This would be more appropriate if a full retaliation would likely render the planet uninhabitable. So, the scale of retaliation would depend on the situation.
I do agree with Farm Boy that the scale of the destruction caused by the atomic bombs in Japan likely was not a surprise. At the very least, there were fairly good estimates of what the destructive power would be from the trial bomb used the previous month. Truman likely knew the general scale of destruction to be expected.
Yeah. If someone is insane enough to order an overwhelming nuclear strike, they have to be checked with at least some scale of retaliation. I agree with Quintillus completely.