Ross Douthat suggests that rather than look to US-Soviet relations, it might be more useful to recall how the world was terrified by the prospect of a nuclear China in the 1960s. There's something to that and, equally, as Ross says the fact that deterrance worked with the USSR and China does not mean that it will always work again. As he puts it, a nuclear Iran is a serious "risk-multiplier". That's why it's possible to be gravely concerned by the implications of a nuclear Iran while also being extremely reluctant to endorse the idea of pre-emptive military action.
Meanwhile, James writes:
Firstly, I suspect that it is not so much a question of being able to "live with" a nuclear Iran but that, eventually, we will have little choice but to do so.“
Diplomacy, sanctions and a blockade should all be tried in an attempt to persuade Iran to give up its nuclear programme. But if it won’t, then the question comes down to which is worse: bombing Iran or a nuclear Iran. Alex says that nuclear Iran is undesirable but that we can live with the risk it represents. But I’m not so sure we can...
Secondly, I'm also unpersuaded by the choice James offers between bombing Iran and a nuclear Iran. Some commenters suggest that opposing attacking Iran means one must be comfortable with the idea of Tehran acquiring nuclear weapons. Not so! On the other hand, some advocates of military action seem quite blithely confident that bombing Iran will actually work. That doesn't seem so very obvious to me. What if attacking Iran doesn't work? And what's the definition of success anyway?
Suppose - hypothetically - there's a 50% chance of an attack setting Iran's nuclear ambitions back by between two and five years. What impact does that have on your calculations of the risk and reward of military action? I'd suggest that the risks outweigh the potential rewards in this instance (though others might disagree). As it happens, I think Iran is likely to be determined to acquire nuclear weapons anyway, but it's hard to see how attacking Iran is likely to persuade them to abandon their nuclear ambitions. On the contrary it might make Iran even more determined to build a bomb - if only to make it less likely that it will be attacked again in the future.
And what sort of military campaign would be necessary anyway? There seems a difference between a single air raid and a prolongued campaign, but each seem quite likely to destabilise the region and risk a full-scale war.
Finally, who benefits from bombing Iran? Notionally but perhaps only temporarily, Israel in particular and the region more generally. But it's also possible that the Iranian regime - unpopular though it may be - would actually be strengthened if Iran is attacked. That is, bombing, even if successful in terms of delaying Iran's nuclear ambitions, might do little to guarantee Israel's long-term security and also advance the interests of the Iranian regime - the very people whom we are attacking. That doesn't seem a terribly compelling bargain.
The risks of inaction and of a nuclear Iran are clear; but it's far from obvious that they are lesser than the uncertainties and "unknown unknowns" that might result from another high-risk military adventure. As we've seen in recent years, it's much easier to start a war than to finish one.