Technically, they have one now. They've lofted a small satellite into low earth orbit. Therefore their missile system has the range to reach the US. Granted, it was a very lightweight payload. I've seen estimates on the web that the satellite was on the order of 50 to 60 lbs. A warhead would be significantly heavier, but then, it doesn't have to achieve orbital velocity either.
Iran continues to improve their missiles, extend their capability. A vehicle that can put a heavier satellite into LEO can easily deliver an even heavier warhead on a ballistic trajectory at ICBM ranges... Make no mistake, they want the capability to strike at their perceived enemies - all of them, including us.
I for one would rather not sit here defenseless in the face of a growing threat. We've put a lot of money into missile defense. Seen some successes, and a few misses. I say lets finish what we've started, and deploy the systems.
Precisely and the weapon to use against the US or Europe would be devices optimized for EMP effects. Such an attack may instantly catapult the entire nation back to the 1800s and any counter attack from us would probably be limited compared to say a direct attack on an American city.