Each time a new nuclear weapons state emerges, we rightly suspect that the world has grown more dangerous. The weapons are enormously destructive; humans are fallible, organizations can be incompetent and technology often fails us. But as we contemplate the actions, including war, that the United States and its allies might take to forestall a nuclear Iran, we need to coolly assess whether and how such a specter might be deterred and contained.
The argument that the Iranian mullahs would be deterred by the risk of American retribution is simply not well-founded. Deterrence doesn't rely on whether the U.S. would in fact trace a weapon back to Iran or whether the U.S. would in fact retaliate in like kind. Deterrence requires that the key players believe that the U.S. could trace a weapon back to Iran and that the U.S. would, in fact, retaliate against Iran if it managed to trace it back. There's good reason to believe that the U.S. wouldn't succeed in tracing a weapon back to an official decision by Iran's government, even if the decision did, in fact, originate from that source. Attenuated connections to "rogue elements" in Iran wouldn't be sufficient to justify large-scale retaliation. Maybe a door just happened to get left unlocked on a storage unit and a Hezbollah operative just happened to stroll in and help himself to a half-dozen "Roses of Mohammed" pastries and a nuclear warhead. Even if the decision to release the weapon could be traced back to official channels in Iran, one can see myriad scenarios in which the "voices of restraint" would argue that the U.S. would have more to lose by retaliation than it would have to gain. Those voices might, in fact, be right. In other words, the chance that the U.S. would ever turn Persia into a big, shiny glass parking lot is fairly low. So long as the U.S. can be counted on to be a rational actor, the threat of nuclear retribution against Iran in the wake of a nuclear attack on U.S. soil (even one traceable to Iran) is probably not credible and the mullahs probably know it. Deterrence is simply not a reliable option against apocalyptic oligarchies.