In response to my statement that I wasn't aware that there was a way of telling the source of a nuclear weapon that had been delivered covertly, a kind fellow named Robert Theron Brockman II writes:

Well, as luck would have it, there is. Every nuclear weapon (and the resulting fallout) has a unique isotopic ratio. This can be linked back to the specific reactor – even the production run! – in which the fissile material was made.

So yes, suitcase nukes DO have a return address – at the very least the address of the dumbasses who let one of their nukes get stolen without having the decency to warn us about it.

Ah, a unique isotopic ration, huh? Well, this is the first plausible response to the question, which I've posed various places.

I'm still curious what our response would be to a nuclear detonation in a major U.S. city, even if we could verify where the weapon was produced. Would we automatically nuke that country?

It seems to me that we just wouldn't have any good options. Look at the anthrax case. Have they still verified where the anthrax originated, with any degree of certainty? It's been over a year since the attacks, and that was powder in a bunch of envelopes.

If a nuke went off in a major city tomorrow, what the hell would we do? Would we wait for a comprehensive investigation, frozen by inaction, waiting for another city to mushroom into oblivion? Or would we begin nuking suspected countries?