When he was running for president, Barack Obama clearly stated his views on a president's authority to order a military attack.
Responding in 2007 to questions from The Boston Globe, he wrote: "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. ...
"In instances of self-defense, the President would be within his constitutional authority to act before advising Congress or seeking its consent. History has shown us time and again, however, that military action is most successful when it is authorized and supported by the Legislative branch. It is always preferable to have the informed consent of Congress prior to any military action."
So by his own standard, Obama should not have ordered U.S. participation in recent military strikes on Libya, because he did not have Congress' consent. The departure of Libyan dictator Moammar Gadhafi from power is desirable, but no one can realistically claim he poses an "imminent threat" to the United States.
Since the Constitution empowers Congress to declare war, the president should have sought a congressional declaration before authorizing a U.S. role in the attack on Libya.