On the subject of impending military strikes against Syria, I have little to add to my prior post on the similar situation in Libya in 2011. I argued then (writing at Opinio Juris):
Every major figure from the founding era who commented on the matter said that the Constitution gave Congress the exclusive power to commit the nation to hostilities. Notably, this included not only people with reservations about presidential power, such as James Madison and Thomas Jefferson, but also strong advocates of the President’s prerogatives, such as George Washington and Alexander Hamilton. As President, Washington on several occasions said that he could not undertake offensive military actions without Congress’ approval. Hamilton is especially significant, because his views on the need for a strong executive went far beyond those of his contemporaries. Yet Hamilton made it very clear that he read the Constitution not to allow the President to begin a war – as he put it at one point, “it belongs to Congress only, to go to war.” (References are found in my article “Textualism and War Powers,” 69 U. Chicago L. Rev. 1543 (2002), in part I.A).
Does the Libya intervention amount to a “war” in constitutional terms? Samuel Johnson’s influential 18th century dictionary defined “war” broadly as “the exercise of violence under sovereign command, against withstanders.” International law writers of the time expressed similarly expansive definitions. Although there are surely borderline cases, our use of force against Libya easily qualifies: the opponent is the Libya government, and our objective is to use “violence under sovereign command” to force that government to change course.
The fact that our use of force is limited to air strikes should not matter. Limited wars were well-known in the 18th century (Britain and France fought a limited war at sea and in North America during the American Revolution). The U.S. fought two limited wars early in its history, against France beginning in 1798 and against Tripoli in 1801. So far as I know, every person commenting on these events at the time thought that Congress had to authorize any initiation of force, even limited naval attacks. (In 1801, Hamilton argued that authorization wasn’t needed because Tripoli, not the U.S., began the war; but he agreed that congressional authorization would otherwise be necessary even in the context of an attack on a single Tripoli warship.)
Thus the founding generation thought the Constitution reserved war-initiation power to Congress. How could this be, though, if Congress has only the power to “declare War”, which we may think refers to making a (now-outmoded) formal announcement? Why can’t the President begin a war informally, merely by ordering an attack, without a declaration?
The answer is that in founding-era terminology war could be “declared” either by formal announcement or by military action initiating hostilities. John Locke’s classic Two Treatises of Government from the late 17th century referred to “declar[ing] by word or action.” Blackstone and Vattel, two of the 18th century legal writers most influential in America, also used “declare” in this way. Vattel wrote: “When one nation takes up arms against another, she from that moment declares herself an enemy to all individuals of the latter.” Johnson’s dictionary gave as one definition of “declare” to “shew in open view” – which, applied to warfare, would obviously encompass military attacks. (References are found in my Chicago Law Review article, Part III; for a more comprehensive account, see this outstanding article by Saikrishna Prakash). Thus in 18th century terms initiating an attack was as much “to declare war” as was making a formal announcement; Congress’ Article I, Section 8 power is not narrowly about issuing formal announcements, but broadly about authorizing the sorts of actions that begin war.
Perhaps, though, the President also has power to declare war (after all, the Constitution expressly says only that Congress has it, not that the President doesn’t, and it could be part of the President’s power as commander-in-chief). Returning to Hamilton, a key passage in his Federalist 32 argued that often constitutional power could be held concurrently by different entities. But, he continued, an exclusive grant of power would arise where concurrent power would be “totally contradictory and repugnant” – that is, when one branch’s exercise of a power would wholly undermine an express grant to another branch. Hamilton didn’t give the example of declaring war here, but it fits his model: war, once launched, cannot be undone without consequences. If Congress’ power is to decide when war should begin, it follows that the President cannot independently launch attacks.
And the Constitution’s drafters expressly described the clause as designed to exclude presidential war-initiation power. James Wilson told the Pennsylvania ratifying convention: “This system will not hurry us into war; it is calculated to guard against it. It will not be in the power of a single man, or a single body of men, to involve us in such distress; for the important power of declaring war is vested in the legislature at large.”
As a result, the founding generation’s views are clear and have firm basis in the Constitution’s text: the declare war clause gives Congress the exclusive power to decide when war should be “declared” – meaning begun by “word or action.” In Libya, President Obama has “declared” a war – a limited one, to be sure, but still a war by 18th century definitions – without congressional approval. That contravenes both the Constitution’s text and the founding era’s consensus understanding.
The short of it is that, in my view -- and I have a high level of confidence here -- the original meaning of the Constitution requires congressional approval for any action that creates a state of war between the U.S. and Syria. Whether very limited strikes designed to destroy chemical weapons stockpiles but not to overthrow the Syrian government or otherwise influence the course of the civil war would qualify is a closer question. But again the Johnson definition of "war" is very broad. And surely we would consider another nation's attacks on our weapons stockpiles to be an act of war, even if purportedly for a humanitarian purpose.
See here for similar thoughts from Ilya Somin.
With Syria, unlike Libya, there is the additional issue of whether strikes would violate the U.N. Charter. I think the conventional understanding of the Charter's text is that they would, and that proponents would have to find some non-textual and non-originalist source of law (perhaps post-enactment custom, or a "living Charter"). On this point, though, it is worth noting a textual point that I think has not been fully explored:
The relevant part of the Charter, Article 2(4), is generally said to bar the use of military force against another nation (subject to a self-defense exception in Article 51, not applicable to the Syria situation). But the actual 2(4) language is more complicated:
All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.
Thus the ban is not on the "use of force" against another nation but on the "use of force against territorial integrity or political independence" of another nation. Can there be a use of force against a nation that is not a use of force against its territorial integrity or political independence? Textualist principles would say yes: (a) otherwise the "territorial integrity or political independence" language is surplusage; and (b) the next clause ("or in any other manner ...") suggests that there are uses of force not covered by the "territorial integrity or political independence" language which nonetheless may be (but may not be) inconsistent with the purposes of the United Nations.
If so, again perhaps a very limited strike on chemical weapons facilities would not be prohibited. It would not be a use of force against Syria's territorial integrity, in the sense of attempting to forcing the surrender of territory (which is probably what the Charter's drafters, acting in the shadow of the territorial conquests of the 1930s and 40s, had in mind). Whether it is a use of force against Syria's "political independence" depends on what that rather uncertain term means. Arguably, an attack that does not attempt to influence the course of the civil war, apart from eliminating chemical weapons, does not fall within the ban; Syria would remain politically independent in the sense of remaining a fully sovereign nation, unlike for example a situation in which a new government is forced upon it by outside arms. And arguably such an attack would not be inconsistent with the purposes of United Nations, if it were only to stop the use of chemical weapons and not to affect the outcome of the civil war.
I am not aware of any comprehensive investigation of the original meaning of Article 2(4)'s text, supporting this view, however.