I don't lie awake at night wondering about the plans of the Iranian Mullarchy to become Death, the Destroyer of Worlds, plunging the world into a sea of nuclear flame. In fact, the thought seldom occurs to me at all, and when it does, I find it somewhat amusing, actually.
There. I've said it.
Now permit me to explain myself. I'd rather that a regime such as that of the Mullarchy not possess nuclear weapons, and on principle. Such regimes, combining several undesirable qualities - Islam, evil, general contumacy - are not the sort with which one would trust such weapons, were there an option. Nevertheless, the development is animated by a certain logic, which would obtain even in the absence of the Mullarchy, and which, moreover, militates against the use of such weapons by the Mullarchy.
The linchpin of this logic? Iran is desirous of becoming the regional hegemon, the dominant power of the Near East, a long-term geostrategic ambition antedating the Mullarchy, which would outlast that regime. Iran could be liberated from the shackles of the Islamic Republic, and Persian nationalist sentiment would keep alive the both the ambition and the nuclear program itself, which would be both symbol and surety of that status, should it be achieved. This regional ambition has two main consequences, as far as the nuclear program and the West are concerned. First, a first-use of nuclear weapons on the part of Iran, whether directly, or by means of proxies - such as Hizbollah - is highly improbable, since the logical conclusion to be drawn in such a case will be precisely that Iran has used/supplied the weapons, and Iran will cease to exist as a functioning, post-medieval state. Israel and the United States will see to that, morally licit or not.
Second, and in consequence of the first consideration, any use of nuclear weapons merely suspected to originate in the Iranian program will entail the permanent demise of Persian ambitions for regional preeminence. And I would suggest that, the apocalyptic rhetoric of a former Tehran traffic engineer aside, the Iranian powers-behind-the-presidency are more than worldly enough to value that ambition over the annihilation of the Zionist Entity. Incidentally, this is the reason for the low probability of Hizbollah being provided the eventual products of a mature Iranian nuclear program. One does not hand the keys to the kingdom to third parties one cannot completely control.
Finally, as a concluding observation, a the development of a nuclear program in Iran is more or less inevitable for another reason: Iranian oil and natural gas production is declining, and I consider it highly doubtful that Iranians will be willing to contemplate a return to premodernity, merely because the only long-term means of avoiding that fate is one of which the West disapproves. This is not to argue that Iran needn't be countered in Syria or Lebanon, nor that such countering need never involve military action of some sort. It is only to argue that a nuclear Iran can be deterred. And, as regards regime change - well, have we learned nothing?