Good question

Are We Really Going To Nuke Iran?
Decoding our options.
By Fred Kaplan

What are we to make of Seymour Hersh's bombshell, in this week's New Yorker, that not only is President George W. Bush keen to attack Iran's nuclear facilities but that several higher-ups in the White House and the Pentagon would like to do so with nuclear weapons?

[...]

Is this for real? Is President Bush or anyone else in a position of power truly, seriously thinking about dropping nuclear bombs on a country that poses no direct threat to the United States, possesses no nuclear weapons of its own, and isn't likely to for at least a few years? 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.

[...]

Another possibility is that Bush is going to launch some sort of raid on Iran, and if people think he might drop nuclear bombs, they'll be relieved—they'll consider it a relatively moderate gesture—if he confines the attack to conventional bombs.

They are masters of managing expectations, we should all be scared.