I have only heard three explanations of why the US would contemplate such a move: 1) a last desparate attempt by the neocons to implement their grand design; 2) a desparate attempt by the Bush admin to cover their failure in Iraq; 3) pressure from Israel to implement their own grand design (essentially, the same as 1) above).
These fantastic, minority and parochial interests seem hardly enough to explain such an important and risky foreign policy move. Can anyone suggest a plausible strategic vision that would be served by this plan? For example, I can see that the Atlantic group might be worried about a Russian-Chinese energy alliance that included Iran, but normally the way to counter that would not be by attacking one of its members.