What is the US supposed to do about an Iran that is developing seriously dangerous weapons?

First, let us assume for this discussion that Iran really is developing the capability to produce nukes, or to supply that technology to others. We have, after all, set the stage in such a way that it makes sense for them to do so. In international politics, having nukes lets you sit at the special table. We've made it clear that if we could do so without fear of repercussion we'd already have invaded. If you don't think so, look at our policies. What's the difference between North Korea and Prewar Iraq? The ability to hurt us and our allies that North Korea has but Iraq did not have.

We know what doesn't work just by looking at the current situation in Iran, Afghanistan, and Lebanon. Military force doesn't change minds. Is there a historically successful model to take as an example? I think there is. Take a look at the long term results of Nixon's policy of engagement with China. China in 1970 was a rapidly developing state with Nukes and a poor relationship with the US on a political level. By engaging with China at whatever level was possible, over time we have built up a partnership in business and an understanding politically. China still falls far short of what we'd like to see in a neighbor and I'm not endorsing their policies here; but I would suggest that United States is much less threatened by China today than it was in 1970.

I believe the way to deal with Iran (and Cuba, but that's another issue) is through engagement. The Iranian people are intelligent, strong willed, and proud. Many of them are very progressive politically and favor western style economic and political models. At present, that large minority isn't in power, but the concepts of tolerance and democracy are there. Increased threats and sanctions, the specter of war, and most of all fear will only help those who push a nationalistic military based repressive society -- on both sides of this issue.