Using nukes on a terrorist organization not really a thing, now say Iran was proven to have given some terrorist organization a nuke which they used on the US in some way, then the US nuking Iranian military sites and their nuke facilities is likely. IMHO the USSR/Russia is not going to make too much noise about the USA responding to a WMD attack with nukes as long as they are not taking out cities - in the Iran case military sites, nuke sites sure, Tehran no (at least not with nukes). The Soviets/Russians are not going to risk putting their male members in the sausage machine defending somebody else who was stupid enough to do something the USA decides to use nukes against.