Let's be very careful, here. In addition to being morally reprehensible, direct deliberate attacks against civilian populations are militarily useless. Unless the destruction of said civilians is the military goal. In which case, one is engaged in genocide. The use of nuclear weapons, and maybe chemical weapons, against military forces or installations, OTOH, certainly can be legitimate. If SH, for example were to barrage our troops with VX, or nuke them, or worse detonate something like that in CONUS I would not be averse to nuking every last one of his hidden command bunkers. We even have a nice earth-penetrating dial-a-yield weapon just perfect for that sort of thing. But the loose talk of "turning Baghdad into a parking lot" is repulsive and beneath us.
However, a large-scale WMD attack against one of our civilian population centers should be met with an identical counter-strike. Otherwise, the enemy can park his military inside population centers and blast away at ours without retaliation.
Genocide, my *#&$& ... fire-bombing Dresden and Tokyo was not genocide. The two nukes at Hiroshima and Nagasaki were the best investments that we made in wartime, saving the lives of tens of thousands, if not hundreds of thousands, of our own troops (and Japanese defenders, civilian and military .. although that doesn't really enter into MY equation).