Check out this excellent piece from Franklin Foer, mainly about the state of neocon thinking on the situation in Iran but also an excellent primer on the difficulties the United States faces there. The money quote, from AEI's Tom Donnelly:

The horse is ninety percent out of the barn. They're going to get the bomb unless we invade. That's not an option. So, I'd say, the time to stop this from happening has pretty much passed. Now, the question is, what are you going to do about it?

There are several points worth making on the Iranian situation. First, wouldn't it have been nice if we'd paid some attention to this crisis in the making? Second, trouble in Iran is collateral damage in the continuing Iraq insurgency (and in continuing Afghanistan instability). How much stronger would our position be if we'd taken Iraq with appropriate strength and quickly pacified the country. With decisive victories in Iraq and Afghanistan we have Iran surrounded, both by democracies and by victorious troops. Instead, Iran has the option of assisting the insurgents, nickel and diming our troops to death, and diminishing everyday the will of Americans to take on another military venture should one prove desirable but not absolutely necessary.

What do we do? Having put ourselves in this situation, I think the best solution is diplomatic. Not only is invading off the table, but from the sound of things a tactical strike against Iran's nuclear facilities would also have more costs than benefits. It seems to me we should work with Europe, and perhaps try to win assistance from Russia or China, to stall Iran's nuclear development as long as possible, and shine an international light on opposition crackdowns. In the meantime, we need to throw everything we have into establishing safe, prosperous, and secure nations in Iraq and Afghanistan. The longer a power vacuum persists in those regions, and the stronger Iran is allowed to become, the more we invite Iran to directly assert its influence, a move that could have catastrophic effects.