The options are negotiate, launch aerial attacks to take out the nuclear program, or to enact regime change through one means or another.
Then the chances of each are outlined.
Negotiations are out since we want to negotiate and end to their nuclear weapons programs and the Iranians are pretty hot on getting nukes. What's to negotiate? In addition, the Israelis are unlikely to just sit by while negotations buy Iran time to build nukes. They'll accept buying time with a strike since Iran already hates them. Nukes next week are better than nukes tomorrow from their perspective. I've noted before that this factor alone--even if negotiations might succeed--pushes us toward a more active approach.
Aerial attacks might or might not work and just as important, since the mullah's regime will still exist, their response is not knowable.
Invading is out since we are tied down in Iraq, Afghanistan, and Korea with our ground forces. We could invade but then we'd have to mobilize all the Army National Guard and everyone would be in for the duration or until new units are created.
Regime change by covert operations might work but encouraging peaceful resistance as in Poland with Solidarity took 8 years to work. We don't have that time unless we assume the mullahs are hell bent on getting nukes fast in order to sit on them indefinitely.
What I'm hoping is that we have been cultivating contacts in the regular Iranian military and dissidents in order to support an Iranian revolution against the mullahs. With even the Pasdaran not fully reliable, the mullahs might not have very much to defend the regime against organized and trained forces attacking them. The US could help with special forces-air power assets. With the public in Iran overwhelmingly against the mullahs, a local effort to overthrow the regime with minor US help hopefully won't inspire nationalistic support for the regime. This alone could change the regime with a speed that does us some good.
And if the new Iran still wants nukes? Well that isn't the best option but they would at least not be a psycho regime with nukes. I actually trust France not to nuke us. Really. At best, the Iranians might after a time decide the cost of nuclear programs is too high to continue. Or they might accept international safeguards that will actually keep nuclear energy programs from being used for weapons. Any of these tracks is better than nutball mullahs with nukes.
We put Iran on the Axis of Evil for a reason and I don't think it is unreasonable to assume we are acting on that. Of the other two states, we invaded Iraq and we are squeezing North Korea which increasingly looks like it is on the verge of collapse. Is Iran uniquely being left alone? I don't think so. Are we willing to let Iran go nuclear? I don't think so.
I expect something to happen in Iran soon.