I think its safe to say that the US, if its the victim, will hit whoever it thinks did it. And if we aren't sure, then we will hit everyone who might have wanted to.

See this: http://www.belmontclub.blogspot.com/2003_09_01_belmontclub_archive.html#106401071003484059

If such an event were to occur (a terrorist attack with a nuclear weapon in Paris 2009) it might provoke France to finally join the war on terrorism. I don't see the US withholding any information or intel that would have tipped the French off to the threat, however. As much as we hate the French opposition to US foreign policy, we'd rather change their attitude through magnanimity than maliciousness. There's far too many Judeo-Christian moralists in the halls of American power to find ourselves give the French what they so satanically deserve. Unless we catch them actively assisting terrorists against us. Then, the nuke in Paris might carry a "made in USA" sticker on it. Who knows?

"Would it trigger a spate of soul-searching ...or would hell break loose " Both. But, mostly the latter is there is any target, at all, to hit. Mmmm...this is a subtle matter, but important. The key concept is the way the psychology of a people can change under the pressure of some defining event or series of events. Take, for instance, the way the American public thought before the Japanese attack on Pearl Harbor, and the way they came to think during the War. Before, the public was deeply divided on almost everything relating to both the situation in Europe and the situation in East Asia. After December 7, the public was as nearly unanimously set on war as it's possible for a large group of people to be. Mindsets changed so quickly and so completely that when Hitler declared war on America - with no obvious way to fight America, and being no obvious threat to anything American apart from ships at sea - the American reaction was almost automatically to devote itself to the complete destruction of Hitler, the Nazis, and the German military, and if that meant the destruction of Germany in the effort, there weren't many who were seriously bothered by the prospect. In a very few years, a people went from the point where most were deeply bothered at the thought of any fighting, at all, even on a purely military-to-military basis to a willingness to literally destroy whole countries. To burn whole cities to the ground, and to keep on doing so until victory, even if the entire enemy nations died in the process. The world saw a LITTLE of how far things can change in the way the bulk of America reacted after 9-11. BUT, ONLY a little. That wasn't America "angry". That was America "seriously annoyed". A nuclear attack, even a "small" one - that is, a real, fission event. Not "just" the release of radioactives. - in an American city? Be afraid. Be very afraid. You's see something the muslims would find indistinguishable from a real, contemporary Crusade and the people held responsible - we're talking an arab or islamic attack, aren't we? - would be treated rather as the Germans and Japanese were in WW2. It wouldn't be a question of changing a few rulers or even governments. The question of the survival of whole states would be on the table. The American people would be demanding blood, in million-gallon lots, the strategic planners would be demanding massive retaliation to instill real fear in potential enemies, and so on. The world would change, dramatically, and irrevocably. Anyone who got in the way, by force, or by verbal opposition, would find themselves in the position of a person lying across a railroad track with a train barreling down on them.

Globally and domestically. HOME AND ABROAD, CL.

A nuclear bomb exploding in a western city already meets the description of hell breaking loose. What comes after is just the dust settling. My guess, if it was in a French city, would be a final realization that this was a war of civilizations, and when the one civilization breeds people who are capable of such acts, the other civiization can no longer allow the luxury of high ideals to imperil its existence. Yeah, there would be noise across the world, comparing the resulting offensive against Islamic civilization to the holocaust of WWII, but when you think about it, if they had annihalated Berlin, maybe the Jews would have deserved the holocaust.