The answer, in my view, depends on whether the U.S. can be “at war” with a non-state actor such as the Sunni rebel force in Iraq. Here’s my analysis:
(1) The President has executive power and commander-in-chief power, and thus can direct the military and conduct foreign relations, so long as he does not do anything the Constitution prohibits or exercise a power the Constitution reserves exclusively to another branch.
(2) The principal limitation on the President’s power, for current purposes, is that the President cannot “declare War” — a power reserved to Congress. (I assume that a strike on rebel military targets authorized — indeed, apparently requested — by the Iraqi government would not violate international law [but see this argument to the contrary] and so I leave aside the question whether the Constitution requires the President to take care that international law is faithfully executed).
(3) Per my prior writing, I understand “declaring” war to include initiating war through armed attacks. Because armed attacks on a sovereign government initiate a war between that government and the United States, the President must have Congress’ approval before launching such strikes. Thus, in my view, the U.S. action in Libya was unconstitutional and the proposed U.S. action in Syria would have been unconstitutional without Congress’ approval.
(4) The complication here is that the target is not a sovereign government, and indeed the operation would have the approval of the territorial sovereign. Thus the proposed strikes would not declare war on any sovereign government. The question, therefore, is whether they would declare war on the rebel forces. And that turns on whether it was possible, in the original meaning of “war,” to have a war against a non-state actor.
(My tentative view is yes — so that the answer is Congress must approve — but I think it is far from clear).
Related: At Lawfare, Jack Goldsmith reaches a similar conclusion by reasoning that seems misconceived to me:
The OLC precedents (here is the latest public one) maintain that the President can use force abroad without congressional authorization if the use of force (1) serves sufficiently important national interests, and (2) has a relatively limited scope and duration so as not to constitute “War” that would require congressional approval. The difficult prong to satisfy in Iraq (and Syria) is (1). Traditionally the “national interest” criterion was satisfied if the President was acting to protect U.S. persons and property. Anything that goes beyond that requires special justification, for if the “national interest” is unmoored from protecting U.S. persons and property and interpreted too broadly, there is no effective legal limit on the President’s unilateral power to use of force, since U.S. national interests are so capacious. …
I simply do not understand where in the Constitution one can find either (a) a requirement that the President act only in support of “important national interests” or (b) a rule that “important national interests” means only “acting to protect U.S. persons and property.” As to the first, I suppose it’s implicit in the President’s office that he act in the U.S. national interest, and as a prudential matter presumably one should not use military force except where the interest is “important,” but the idea that the Constitution imposes some sort of “importan[ce]” requirement seems unmoored to any text. The second claim seems even less based in constitutional text (and I suspect, despite Professor Goldsmith’s suggestion, in history). The power to command the military is not textually limited (apart from the limit on starting wars), so while it might be a good idea to limit the President’s use of the military to protecting U.S. lives and property I don’t see anything that does so.
Professor Goldsmith says that without his limit, “there is no effective legal limit on the President’s unilateral power to use of force” — but that’s not true: the textual limit is that the President cannot use military force to start a war.
NOTE: This post was originally published at The Originalism Blog, “The Blog of the Center for the Study of Constitutional Originalism at the University of San Diego School of Law,” and is reposted here with permission from the author.