Matt Duss has an excellent piece for the Prospect explaining why military action against Syria is probably a terrible idea on policy grounds. In addition to the question of whether the policy is wise, however, it's worth considering whether a unilateral decision to attack Syria by the president would be legal.
At the outset, I should make clear that I'm talking purely about legality under domestic law; I'll leave the question of whether military action against Syria is justified under international law to others. I also don't subscribe to the the most formalist conception of the president's military power, which holds that any non-emergency action by the president requires a congressional declaration of war. Military action accompanied by a congressional authorization for military action (as with the second Iraq War) should be considered clearly constitutional, and I'm inclined to think that presidential initiations of military force in the face of congressional silence are presumptively constitutional, although there are very credible arguments to the contrary.
In this case, however, Congress is not silent, at least before the fact—the War Powers Resolution is clearly pertinent to a potential attack of Syria. Even assuming that the president will meet the act's requirements to notify Congress, this leaves us with several important questions:
Is the War Powers Act Unconstitutional?
Every president since the War Powers Resolution (WPR) passed in 1972—after Richard Nixon's veto was overridden—has asserted that the law is unconstitutional. (Obama, however, has not taken an official position.) I disagree with this assertion. As suggested by my previous argument, I generally think the separation of powers should be interpreted to provide flexibility rather than rigidity, a position that has the advantage of better reflecting how separation of powers works in practice. The executive branch can't have it both ways: If congressional delegation of war powers to the executive is constitutionally acceptable, then surely it's also acceptable for Congress to place limits on this delegation. War powers are shared between the executive and legislative branches, and the WPR does not unconstitutionally infringe on any Article II powers.
Would an attack on Syria without congressional authorization violate the War Powers Resolution?
There are two potential arguments that a unilateral attack on Syria would not be inconsistent with the WPR. The first, which the Obama administration used to defend its unilateral decision to use military action against Libya, would be that the action does not constitute the "hostilities" that trigger the 60-day clock. Without knowing what military action would consist of we can't know if this argument would be plausible, although it seems unlikely.
The more substantive defense would cite Section 5 of the Act, which says that when the president sends armed forces "into hostilities or into situations where imminent involvement in hostilities is clearly indicated by the circumstances" the president has "sixty calendar days" before he is required to start withdrawing them if congressional authorization has not been obtained. (There is then a 30-day withdrawal period.) In isolation, this would seem to suggest that the president can unilaterally initiate military action consistent with the WPR, although he may not be able to sustain it without congressional approval.
Looking at the statute as a whole, however, complicates the question considerably. Section 2 of the WPR states that the president's Article II war powers "are exercised only pursuant to (1) a declaration of war, (2) specific statutory authorization, or (3) a national emergency created by attack upon the United States, its territories or possessions, or its armed forces." The most natural reading of the statute is that the 60-day period in Section 5 only applies with the president initiates force in response to an attack on the United States or American troops.
If this is the case, then the president cannot take unilateral action, as Syria poses no imminent threat to the United States or its troops.
How can this ambiguity be resolved?
The courts are enormously unlikely to address the question of how the WPR should be construed. This leaves it up to Congress to decide whether to assert its powers or not. And as MSNBC's Adam Serwer notes, Congress is highly unlikely to defend its prerogatives. For both strategic and policy reasons, Congress has been more than happy to defer to the executive when it comes to its own military powers. Ultimately, the WPR means what Congress says that it means, and in practice its actions have ratified a very latitudinarian conception of the president's power to unilaterally initiate military force. It's hard to see why a decision by Obama to attack Syria without congressional approval would play out differently.
Whatever one thinks of the constitutional issues, Congress's abdication of responsibility is a bad thing. The current institutional equilibrium has led to a perverse place where it's enormously difficult for the president to appoint people to fill minor executive branch positions but he can bomb anything he likes with almost no prospect of congressional pushback. This is the wrong way around. Even if Congress thinks it's washed its hands of responsibility through inaction, the legislative body shares the blame if there's an attack on Syria that goes badly.