Barack Obama has "dramatically expanded" the notion of when presidents can use force without permission. He has left "an extraordinary legacy of war powers." History will assign far more importance to these precedents than we do. They make it significantly easier for future presidents to wage war unilaterally.
Those may sound like the concerns of an anti-war activist. In fact, they're the conclusions of Jack Goldsmith, who led the Office of Legal Counsel for part of the George W. Bush administration. He isn't someone with a narrow understanding of executive power. But in a recent speech, he cited three specific ways that Obama expanded the war power beyond anything attempted by the Bush administration.
The three precedents are as follows:
1) Article II, Section 2 of the Constitution declares, "The President shall be commander in chief of the Army and Navy of the United States, and of the militia of the several states, when called into the actual service of the United States." Almost everyone agrees that this gives the president the power to repel an attack on America. President Bush argued it empowered him to preempt imminent threats.
Obama has gone a step farther.
He cited his Article II authority to justify waging war in Libya, a "pure humanitarian intervention" with "no conceivable self-defense rationale." And while he did not bomb Syria after its regime used chemical weapons, he argued at the time that Article II empowered him to take action unilaterally in order to "protect regional stability" and "enforce international norms," a standard that would permit an incredible variety of unilateral wars. As Goldsmith put it, "to decouple the use of force in such a clear way from self-defense" is a sweeping change.