Don Mike,
That's why I advocate dropping a nuke on their own nuclear testing site. They already cleared it, hopefully at least. I'd like to hit it while the ground at ground zero is still molten. Preferably we'd hit it with a twenty megaton bunker buster, placing the blast underground so as to minimize the blast radius. Also, it might be noted that the fallout from Russian and US thermonuclear warheads is fairly negligible, as unconsumed nuclear fuel is lost potential explosive power and the fission fusion cycle tends to burn more uranium or plutonium.
My preferred impact is political, leaving civilians out of it. A nuclear test is generally a threat to one's enemies. A government is likely to bask in the national pride following a nuclear test for some time. Upstaging that government in a flashy nonlethal way might be the best way to cut the public support for their nuclear program. Given a situation where I might have my finger on the trigger I'd like to sit down with a lot of intelligence agency people, international political analysts, and military figures before making it policy. It's a good idea theoretically to expend 20 megatons without hurting anyone in response to a threat, but so was being greeted as liberators, so I don't think I'd rush into it or limit my opinions to yes-men.
In Iran's case, their fielding of a nuclear weapon would probably bring a much more terrifying response from Israel, which might make my conjectures a moot point.
My preferred present day philosophy would be to lift sanctions on Iran, and do my best to deliver satellite TV and unfiltered internet, letting market forces erode their oppression. I'd also like to apologize for past infractions against Iran, like the thing were we installed the Shah. Nothing hurts an oppressive regime like taking away the great satan they're united against.
So yes, greatest respect to the Iranian people, but strong shows of force might keep us from having more nuke having nations in the future, and that's worth some political risk. We're presently outside of North Korea's range, but I'm sure South Korea and Japan aren't too thrilled about nuclear weapons being wielded by their erratic oppressive neighbor.
I've clearly given too much thought to destabilizing oppressive regimes and halting nuclear proliferation today.
For the record, I also would have tried to buy Iraq from Saddam Hussein before invading. Not that I supported invading it, but it wouldn't have hurt to have made the offer, before tearing the country apart.
I think I'm derailing this thread.