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).
The bombing campaign against Japanese cities was somewhat better justified in that their war industry was spread throughout the cities. The atomic bombing of Hiroshima and Nagasaki caused "medium sized" damage, compared to other air raids at the time, and certainly destroyed the war industries and port facilities of those cities. A nuke is as much a WMD as a fleet of bombers full of napalm.
If our opponent (Saddam Hussein, for example) decides to surround his legitimate military targets with civilians, their deaths are on his hands, not ours. There's a big difference between attacking your opponents' army (and accidentally killing civilians) and deliberately targeting civilians for the sole purpose of killing civilians. If one is engaged in the latter, one is engaged in mass murder. SH did precisely that to the Kurds in the early 1990s; it's one of his many crimes for which I'd like to see his head on a pike.