In 2007, Barack Obama was asked when Presidents have the authority to launch a military strike without congressional authorization. He had a precise answer at the ready.
“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,” Obama told the Boston Globe.
As reported by NBC News, as a presidential candidate in 2007, now Vice President Joe Biden threatened to impeach President Bush if he unilaterally attacked Iran. “And I want to make it clear, I want it on the record, and I want to make it clear, if he does, as chairman of the Foreign Relations Committee and former chair of the Judiciary Committee, I will move to impeach him.”
http://swampland.time.com/2013/08/27/syria-intervention-would-reaffirm-obamas-biggest-flip-flop/#ixzz2dGMsL9PD