The point about small-country or terrorist nuclear weapons - of either the 'dirty' or 'big bang' types - is that their use would wonderfully concentrate the US's attention on the user. This has not, in the past, proven to be a good idea.
Muslim countries are walking a fine line, taking up unrestricted warfare on our civilians and cities; but since most of them are ignorant of history past the 14th Century, and are illiterate in any language but their own to boot, they invite a kind of warfare that they cannot even envision.
The Germans and to a lesser extent the Japanese introduced the theory of "unrestricted warfare" in WWII, to their eventual regret: They forgot that the United States invented "scorched earth" warfare during the Civil War. (Sherman's march to the sea) The Germans never even managed to destroy London, even using everything from manned bombers to the precursors of Cruise Missiles and ICBMs. The US became City-Killers beyond their wildest dreams, which essentially put paid to their ideologies.
On the other hand; Iran, Syria, or even Saudi Arabia contain no cities that even rate a 10-kiloton pony bomb. They manufacture nothing, they make nothing, there are no industrial centers that could benefit from the application of a nuclear weapon. (Well, perhaps Teheran, where they are attempting to build one, and a single 20-megaton would solve that problem, likely forever)
The thing to remember about these people is that they are the ultimate parasites, technologically. H*ll, if Toyota stopped selling them trucks and SUV's, they'd be as militarily effective as, say, the Comanche were. The much more efficient method of dealing with them would be using Fuel-Air Explosives and the occasional MOAB. This would have the additional advantage of being environmentally friendly.