I can answer the question Senator Webb put to Secretary Rice yesterday. The answer is, yes, it is "the administration's position that it possesses the authority to take unilateral action against Iran in the absence of a direct threat without congressional approval." They haven't yet directly said "we can launch a war with Iran and we don't need anyone's permission," but it's not hard to read between the lines:
In April 2002, John Yoo, then with the Justice Department's Office of Legal Counsel, testified before the Senate Judiciary Committee that "the President has the constitutional authority to introduce the U.S. Armed Forces into hostilities when appropriate, with or without specific congressional authorization.” In an internal memorandum prepared shortly after September 11, 2001, Yoo had put it even more starkly: “In the exercise of his plenary power to use military force, the President’s decisions are for him alone and are unreviewable.”
That is consistent with Vice President Cheney’s long-held view of the president’s powers, as can be seen in this Frontline interview in which Cheney discusses his role as secretary of defense during the Gulf War:
Q: The Congressional vote. Do you recall discussing with the President what he would have done if he'd lost the votes.
Cheney: It was my view at the time [that] we were absolutely committed to getting Saddam Hussein out of Kuwait one way or the other, no matter what we had to do. We had to have the Saudis as allies in that venture, but if no one else had been with us, if it had just been the United States and Saudi Arabia, without the United Nations, without the authorization of the Congress, we were prepared to go ahead. I argued in public session before the Congress that we did not need congressional authorization.... I was not enthusiastic about going to Congress to ask for an additional grant of authority. I was concerned that they might well vote NO and that would make life more difficult for us, or that even if they voted YES and then we had a disaster on our hands and it didn't work, they'd still be against us....
Q: But if you'd lost the vote ...?
Cheney: If we'd lost the vote in Congress, I would certainly have recommended to the President we go forward anyway.
If and when Secretary Rice provides her promised written answer to Senator Webb, I doubt that it will say: "The president has the constitutional authority to launch wars at will, anywhere in the world, at any time of his choosing, without anyone's permission. So yes, of course he can start a war with Iran." But that is the administration's view.
(Hat tip: Matt Yglesias)