Who would think of nuking the US?  China depends way too strongly on us economically (at least for now), Russia isn't really a player anymore, North Korea's little dud wouldn't do squat compared to a themonuke, and Iran probably doesn't have anything.  Even if they do, they are being watched far too closely to try to get away with it.  What's left, terrorists?  The best they could hope for is an old Soviet model sold on the black market, and the US scooped most of them up.
If a nuke is used against the US, Canada, Britain, etc., I would not be at all surprised to see the aggressor wiped off the face of the planet by morning, nuclear retaliation or no. Even the US's conventional arsenal could level several cities.  And if terrorists really do get their hands on a nuke, what happens when Mecca is a pile of radioactive dust?