Going to war without Congressional approval used to be an impeachable offense

Just ask Joe Biden. Before Joe was Vice President, he used to believe that if George Bush were to attack Iran, it would be grounds for immediate impeachment. He was chairman of the Senate Judiciary Committee, taught separation of powers in Constitutional law, convened a group of Constitutional experts to study the subject in 2007, and concluded that:

The President has no Constitutional authority to take this nation to war against a country of 70 million people [Iran] unless we're attacked, or unless there is proof we are about to be attacked. And if he does...I will move to impeach him.

Barack Obama used to agree with his good buddy Biden:

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.

So what has changed since 2007? Maybe Obama and Biden believe that UN approval of a war act supersedes Congressional approval? Maybe they have secret intelligence that Libyan troops are massing on our borders? Or maybe they realize that the legislative branch of our government has surrendered so much of their power to the executive branch over the years that they can thumb their noses at Congress with impunity? My guess is it's the later.