At the Originalism Blog, Prof. Michael Ramsey, a leading academic expert on constitutional war powers, has an excellent post on the implications of the original meaning for the constitutionality of an attack on Syria without congressional authorization (quoting, in part, from a 2011 post he wrote during the debate over the Libya conflict):
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….”
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….
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…
As Michael recognizes, it’s possible that a very small-scale attack could be launched without requiring congressional approval, because it wouldn’t be large enough to qualify as a “war.” I made a similar point here. But at least as far as the original meaning is concerned, any large-scale offensive action against Syria must have congressional authorization.
Obviously, originalism is not the only theory of constitutional interpretation, and some people argue for a broader interpretation of presidential power to initiate war on various nonoriginalist grounds. I addressed some of these types of arguments in a 2011 post on the Libya intervention.