You have been elected president in November. It's now January and you've been sworn in. Iran says they're going to develop nuclear weapons. What do you do?
OK, let's try it on.
Step One: Assess the nature of the threat.
Generically speaking, there are three primary possibilities:
1) It's a bluff/propaganda piece; Iran is posturing for effect within the geo-political context of its region.
2) Iran is serious about developing nuclear weapons but lacks technology for production of weapons-grade uranium and/or a delivery system.
3) Iran is serious about developing nuclear weapons and has reasonable prospects of producing weapons-grade uranium and a delivery system.
These possibilities are complicated by what the current and long-term political situation in the region, especially: Nations with a potential interest in seeing Iran acquire offsetting strength to stand off the US and Israel in the Middle East (right now I'd guess Syria, Palestine, China, and Russia). Our state of relations with those nations and their agendas (Russia v. Georgia; Iranian-Chinese oil agreements, etc.). This is critical to determining assistance that Iran would receive in cases 1 and 2 above.
There is a further point of assessment that only rarely makes it into public political discourse, but which is critical in intelligence circles (and still the prospect of much debate): What motivates Iran to possess nuclear weapons and what use do the country's leaders intend to make of them?
The two major options here follow the George Kennan/William Appleman Williams debate in the early Cold War years regarding the sources of Soviet conduct. Either the Iranians are interested in acquiring nukes because they are fanatical religious nuts who want to use their weapons (or the threat thereof) to pursue an aggressive foreign policy in support of religious/ideological aims, or....
The Iranians are acting primarily out of nationalism rather than religion, and out of fear of invasion rather intent to aggress. In this model, Iran's support of terrorism is the foreign policy equivalent of Cold War surrogate involvement to keep threatening regions too destabilized to become a threat to Iran's territorial integrity.
Step Two: Recognize that there are models of proliferation management beyond the small nuclear club idea that we have been pursuing vainly for the past seven decades. It hasn't worked. Pakistan has the bomb. India has the bomb. Ukraine and Belarus inherited the bomb; there is a statistical chance that Georgia has at least one (a topic that US intelligence sources acknowledge but would prefer not to discuss). South Africa and Israel undoubtedly possess the bomb or (in SA's case) the necessary technology to create one in short order. France, England, Russia, China.... Germany and Japan could possess one in two years or less if they so decided.
Saudi Arabia could purchase a bomb on the post-Soviet market at any given moment.
This doesn't even touch dirty bomb technology, which is inherently easier to acquire and more problematic in terms of proliferation.
But back to Iran:
Step Two: Assess our options.
1) Bilateral diplomacy plus or minus sanctions--unlikely to be productive.
2) Multi-lateral diplomacy plus or minus sanctions but outside the UN umbrella; pretty much the current approach, although we nod toward the UN from time to time.
3) A UN-centered approach (about as likely to achieve our ends as the Oil for Food program in Iraq was to feed starving children).
4) A surrogate military option (Israel bombs the crap out of Iran and asks for forgiveness): frankly, as much as it is the stuff of thrillers, unlikely to be effective and certainly not in our long-term interests if we seek a stable Middle East.
5) The limited use of US military force in surgical strike mode to do the job ourselves; above arguments apply.
6) Invasion: look at how well that's worked out for us in the past. And don't kid yourself: an invasion and even partial occupation of Iraq is a military/strategic type problem that sends generals into early retirement at Arkham Asylum. We have the wrong force mix, the wrong logistic base, and absolutely no willingness to support another long war--and the Iranians know it.
But, wait, the options only end here IF--and it is an incredibly important IF--we assume that US foreign policy is centered around the unquestionable premise that an Iran with nuclear weapons is an unacceptable option in any possible scenario.
Yeah, you read that right.
Because our real objective is generally misstated in political discourse because you have to sell it in Peoria.
Our real objective is an Iran that is not a threat to its neighbors, not a threat to Israel, and not a threat to world stability.
Ironically, Iran today is all of these even without nuclear weapons.
How do we move toward the real objective in the face of an Iran publicly determined to possess nuclear weapons?
First, we follow Hillary Clinton's lead, and practice deterrence on a major scale: we indicate publicly that the use of nuclear weapons by Iran will result in immediate and massive retaliation in kind. The mullahs in Iran are old men who--if they were truly jihadists of the bin Laden ilk--would already have gone out and strapped on the explosives. They have their hands full running their own country, and while they want to be respected and even feared in the region, they have absolutely no desired to be nuked into a plain of glass.
And all propaganda aside, they know we can do it.
Second, we do what Americans do best after long tedious wars: we go home. Remember the dominoes in Vietnam? We'd leave and the commies would be in the Philippines and then Australia before we knew it. Yep. Sure worked that way. Or that Nicaragua was going to become the basis for Central America turning into a Cuban-style Soviet outpost. And so on.
Difficult truth: Other people's long-term plans don't work out any more efficiently than ours, except in Tom Clancy novels.
We need to walk away from the fantasy that a US garrisoned Middle East is conducive to our long-term strategic safety. There is no data to show that it is. We need to stop propping up the Saudi regime. All the little states in the Gulf--from Kuwait to Qatar to the Emirates--have the economic and political clout to negotiate their own survival, and it is not in Iran's best interest to want to annex them, because they are funneling money into Teheran now much more efficiently that they ever could if occupied.
We need to do what we never did after World War Two, but what we have always done after other major wars in our history: get our ground forces the hell out of the Arabian Peninsula.
You know the two primary reasons we're not doing so?
1) Because of corporate profits.
2) Because that's what Al Qaeda has announced it wants us to do.
If Al Qaeda and the Iranians want the entire Middle East from Syria through Afghanistan, let's give it to them. They lack the infrastructure and the trained population to run anything more advanced than the late 19th Century with a few satellite links on the roof.
We need a kick in the ass to get out of the oil dependency business, anyway, and we actually have to ability to do so. China, India, Russia, Japan, and Western Europe cannot bank on developing alternative fuels or converting to natural gas. We can. So let's let the market regulate who sends troops into the Middle East. We don't HAVE to do so.
[China, by the way, understands this, and has been heavily investing in African-oil-rich nations like Angola and Nigeria for the past two decades as a hedge.]
To be blunt: we're due for a good solid decade of military non-interventionism in order to get our own house in order.
So the upshot of my advice to the new President (even for a hypothetical I can't bring myself to believe I'm going to be elected):
Don't panic and keep the public posturing to a minimum.
Practice deterrence, followed by military disengagement, and a commitment to non-interventionist foreign policy.
[Oh, by the way: none of this will happen if your hypothetical comes true, since our military/industrial complex that is part corporate/part government absolutely requires the existence of enemies to thrive.]
That's my answer, Duffy. I doubt you'll agree with it, but you have to admit I didn't dodge the question.