This has got to be a bluff.  If American officials start throwing nukes around, there will be no "coalition" -- and, by "coalition," I assume we mean Britain.  Tony Blair wouldn't dare to support Bush on this.  The Congress and the Commons would both turn on the executives.  The Democrats are already going to pound Bush into the ground for this.  (If they refuse to, the base will walk away.)

Bush doesn't have the troops to invade.  He can't start a draft, because it would never make it through Congress (and the Supreme Court would tell him to go to hell -- yes, even the right-wing justices).  Iraq is a mess.  The entire world doesn't trust America, and it will refuse to go along with this.

His hands are tied.  This is nothing but a political play to get Iran back to the negotiations.

Be nice to America. Or we'll bring democracy to your country.