In 1986, the US waged an undeclared proxy naval war to deter Iran from attacking oil tankers in the Persian Gulf. The US won of course and Iran lacked any effective riposte. This US operation played a decisive role in compelling Iran to accept peace in the Iran-Iraq war.
And it may have prompted Iranian leaders to decide: We need an effective counter-deterrent against the US. The US would have been much more reluctant to protect Kuwaiti tankers against a nuclear Iran. An Iranian nuclear bomb would act as a "Keep Out" sign to frighten the US away from a now truly Persian Gulf.
In other words, an atomic bomb would serve Iran's hegemonic ambitions rather than its apocalyptic fantasies. It is a useful weapon sought by rational people. That is precisely why it is dangerous and must be stopped.
It would be very nice if we could have an enlightened debate framed in this way. Clearly, insofar as Iran is a despotic, illiberal regime and an enemy of the United States, a nuclear Iran is a highly undesirable prospect. The question of whether that prospect--of an Iran one step closer to regional hegemony--is so unacceptable as to merit, as a last resort, a preemptive American military strike, which also comes with huge downside risk, should be at the forefront of public discussion, not to mention the presidential campaign.
Instead, we have a situation in which the vice president of the United States goes on Hugh Hewitt's radio show and tries his hand at deciphering Shiite eschatology in order to suggest that Iran's leaders might, in fact, be suicidal maniacs after all. (And hey, if there's a one percent chance that they are...) To put it mildly, this does not foster the sort of reasoned discourse we need. Under these circumstances it becomes easier to understand why some liberals tend to view any discussion of a military response to Iran's nuclear ambitions as irresponsible saber-rattling.