“The president does not have power under the Constitution to unilaterally authorize a military attack in a situation that does not involve stopping an actual or imminent threat to the nation." — Sen. Barack Obama,
interview with the Boston Globe published Dec. 20, 2007
(...)
Boston Globe reporter Charles Savage (now with the New York Times) asked each presidential candidate a simple question: "In what circumstances, if any, would the president have constitutional authority to bomb Iran without seeking a use-of-force authorization from Congress? (Specifically, what about the strategic bombing of suspected nuclear sites — a situation that does not involve stopping an IMMINENT threat?)"
Obama’s response included the statement above, and he clearly was approaching the issue with the mind-set of a member of Congress, not of a president.
National Security Council spokesman Tommy Vietor noted that, with Libya, the Senate had unanimously passed a nonbinding resolution calling for the United Nations to impose a “no-fly zone” on March 1. On March 18, he said, Obama briefed congressional leadership on the mission he had in mind, and followed up with a letter regarding the commencement of the military operations.
“A mission of this kind, which is time-limited, well-defined, and discrete, clearly falls within the president’s constitutional authority and thus congressional authorization is not required,” Vietor said. “There is ample precedent for such action, including the airstrikes in Bosnia in 1995 that took just over two weeks and involved over 2,300 U.S. sorties, or the deployment of U.S. forces to Haiti in 1993. This is entirely consistent with what the president said in the 2007, as he was referring to an action like we saw in Iraq.”
Full story:
http://www.washingtonpost.com/politics/fact_checker_presidents_war_authority/2011/04/01/AFX3o1PC_story.html