Pre-emptive war—attacking a country to keep it from attacking us or an ally—is sometimes justifiable. Preventive war—attacking a country to keep it from developing a capability to attack an ally sometime in the future—almost never is. And preventive war waged with nuclear weapons is (not to put too fine a spin on it) crazy.
In the 60 years since, the world has declared and observed a clear threshold between the use and nonuse of nuclear weapons. To violate that threshold—for a purpose that falls far short of pre-empting an imminent threat or protecting our national survival—would not only be immoral; it would incite outrage across the Middle East and the Muslim world; it would inspire vast recruitment drives by anti-American terrorists (and any resulting sequels to 9/11 would be seen, even by our friends, as just deserts); and it would legitimize nuclear weapons as everyday tools of warfare and spur many nations into building their own arsenals, if just to anticipate and match their neighbors' impending arsenals.
In short, it would be a disaster of head-spinning proportions.
Of course, Kaplan wonders if this is chess playing by either the Bush administration (play the madman, so that cooler heads are forced to opt for the policy you were gsming for -- in this case, tougher sanctions), or by military officials (to make a conventional attack look like a gesture of goodwill or to derail the Dr. Strangelovian plans in the deep of the Pentagon). Unfortunately, I'm not inclined to think Bush has suddenly learned tatical diplomacy -- he doesn't seem to plan his next move when it is his turn.
So, my answer to Jason Spalding of Just Curious at JadeCastle.com, attacking Iran with nukes would be absolutely worse than doing nothing. It will lead to everybody else to launch nuclear programs, and simply destabilize relations between every single nation in the world.
PS: Bush today said he's not planning on going to war with Iran. He pretty much said the same thing with regard to Iraq, the entire time he was planning on going to war with Iraq -- possibly going so far as suggesting to Blair that we should provoke Iraq into attacking us.