What Does the US Do if Attacked and No One Is Responsible?

Belmont Club imagines a scenario in which there is no one we can threaten with retaliation:

But unattributed retaliation against terrorist sponsors only works as long as the terrorist sponsors are sole producers of nuclear weapons. If fanatical cultists like Osama Bin Laden had an independent capacity to manufacture nukes, no regime of proportional retaliation would result in stable deterrence for reasons described in the Three Conjectures. The murderous cycle of retaliation would continue until someone willed himself to do the unimaginable.

Consider a case where Islamic terrorists obliterate a city, causing five times the deaths at Hiroshima and an American limited response. ... In a war between nations, the conflict might stop at this point. But since there is no one with whom to negotiate a peace and no inclination to stop anyhow, the Islamic terrorists will continue while they have the capability and the cycle of destruction continues. ... At this point, a United States choked with corpses could still not negotiate an end to hostilities or deter further attacks. There would be no one to call on the Red Telephone, even to surrender to. In fact, there exists no competent Islamic authority, no supreme imam who could stop a jihad on behalf of the whole Muslim world. Even if the terror chiefs could somehow be contacted in this apocalyptic scenario and persuaded to bury the hatchet, the lack of command and control imposed by the cell structure would prevent them from reining in their minions. Due to the fixity of intent, attacks would continue for as long as capability remained. Under these circumstances, any American government would eventually be compelled by public desperation to finish the exchange by entering -1 x 10^9 in the final right hand column: total retaliatory extermination.

Islamic terrorists which do not represent states but only themselves; which consist of groups answerable to no more than a handful of fanatics yet have the power to kill millions can only be met — if they can be met at all — by the most radical form of excision. There is no stable plateau at which the process of retaliation would naturally halt. There are no ledges on which the falling stone of destruction will catch. Stopping the serial destruction of the world's cities would require a response so dreadful and so unthinkable that almost any effort to win the war on terror to prevent this fantastic exchange would be well worth it.