Would he still have the authority you claim?
The President, as the Commander in Chief has the authority to attack any threat (in that case Iran) without congressional approval. He only has to notify them. He can use military force 60 days long. After that Congress either approves to continue the action, by declaring war, or it asks the President to cease the action.
The point would be: can we destroy Irans military and WMD capacities within 2 months? We crushed Saddam in 3 weeks... so what are the odds?
Do you think that FDR knew about the specific attack on Pearl Harbor days, perhaps weeks, before it happened? That is my guess, that FDR did know. Yet he dared not launch a preventive strike against the Jap fleet -- why? Because had he done that the worst thing that could happen would have been the destruction of that fleet. His critics and opponents would have forced the war to be limited. They would have strenuously argued and been potent -- with heroic figures of the time, men like Lindbergh and Ford, against it.
Today the danger from nuclear missiles, container-cargo nukes, mines and subs that Iran could launch is a many levels greater than a carrier fleet of WW II. A Commander making a decision to hold fire until struck will have hundreds of thousands of first strike casualities for which to answer.
However, once a preemptive counterstrike is launched, and successful that very success could bring about the impeachment and trial for war crimes of that same commander.
A very hard choice.