My longstanding view, developed in a series of articles, is that the Constitution's original meaning requires Congress to approve any material initiation of military hostilities by the United States. As explained at length in Textualism and War Powers, that conclusion rests principally on two points:
(1) the original meaning of "declare" war includes both formal announcements of the initiation of a state of war and the use of military force in a way that creates a state of war. In the eighteenth-century sense, war could be "declared" by words or by actions (and indeed, wars in the eighteenth century and earlier were often not begun with formal announcements but simply by launching military action — a point noted by Hamilton in The Federalist).
As a result, the Constitution's assignment to Congress of the power to "declare" war gave Congress power over the decision to go to war, whether through formal announcement or by the use of force. A wide range of leading members of the founding generation — including Washington, Hamilton, Madison, and James Wilson — described Congress' power to declare war as exclusive (that is, that it was a power of Congress and therefore not a power of the President).
(2) the original meaning of "war" broadly included most uses of sovereign military force, including ones with limited scope and objectives. An early Supreme Court case, Bas v. Tingy, recognized that there could be general war or limited war — both of which came within Congress' war power. The Bas case involved the so-called "Quasi-War" between the U.S. and France in the late 1790s, which consisted only of naval engagements. Notably, essentially everyone at the time — including advocates of presidential power such as Hamilton and President John Adams — thought the U.S. actions in the Quasi-War needed to be authorized by Congress (which they were).
Applied to the U.S. airstrikes on Iran, this reading seems to require congressional approval. The U.S. strikes constitute war in the original constitutional sense of the term: they are a use of force against a foreign sovereign adversary to compel an outcome. Although their objectives may be limited to forcing Iran to end its nuclear program, such a limited military objective still constitutes a war (albeit a limited war). And initiation of war, whether general or limited, and whether done by formal announcement or simply by the use of force, requires Congress' approval under the Constitution's declare war clause.
The President has two main originalist responses, and a nonoriginalist one. The first is that the strikes are not"war" in the constitutional sense because they are limited in scope and duration and do not involve material risk of U.S. casualties. This was the Obama administration's principal defense of President Obama's somewhat analogous airstrikes against Libya (see Constitutional War Initiation and the Obama Presidency). While there are undoubtedly some uses of military force that don't amount to even limited war, such as rescuing Americans abroad, neither the Iran nor Libya situations fit that category. A limited objective is not sufficient to make a military action not a war for constitutional purposes, as eighteenth-century terminology and the experience of the Quasi-War show. The fact that an objective can be accomplished quickly and with limited casualties may make it a short war, but it is a war nonetheless. The central question is whether the United States is using military force to impose its will on a foreign sovereign — which describes the situation in both Libya and Iran.
The second originalist response is that Iran started the war long ago. There is some force to this claim. It has been recognized since the founding that the President has independent power to use military force to respond to attacks on the United States. Hamilton explained, in the context of the U.S. military actions against Tripoli during Jefferson's presidency, that when another nations declares war (by announcement or military action) against the United States, that act creates a state of war; the President can then use force to respond to the other side's declaration without the need for Congress to declare the war (although Congress often does so).
It's disputed how far the President may go in response to an attack. My view is (with Hamilton) that the President may fully engage the attacker, as explained in The President's Power to Respond to Attacks. But others, including leading presidential power scholar Saikrishna Prakash, have the much narrower view that the President may respond defensively but not take counteroffensive actions without congressional approval.
Obviously the narrow view would not support the President's actions against Iran. Even if one thinks that the President can take preemptive action to defend against an impending attack, there was no impending Iranian attack on the United States. On the broader view, the President might plausibly argue that a long line of Iranian-sponsored terrorist attacks on U.S. forces and interests amounts to a declaration of war against the United States by Iran, to which the United States is now belatedly responding. This is, at least, a better argument than President Obama had regarding Libya, whose leader had renounced hostile intent against the United States. On balance, though, I find it unpersuasive. Iran had not launched direct attacks against U.S. interests in recent years, and the United States had not treated earlier attacks as acts of war (even if arguably they were). While there have been rumors of Iranian plots against the United States and the President, these seem unconfirmed and in any event seem better understood as potential justifications for initiating U.S. action against Iran rather than war-creating acts in themselves.
Thus I conclude that under the Constitution's original meaning the President's action against Iran (like President Obama's action against Libya) required congressional approval. To be clear, that doesn't mean I think it was a bad idea. I did think Obama's attack on Libya was a bad idea, but the action against Iran seems much more justified in terms of U.S. strategic interests. The point of the declare war clause, however, is that Congress is supposed to make the assessment of the the U.S. strategic interest (and could easily have done so here given the longstanding threat from Iran).
Finally, the above discussion proceeds from an originalist perspective. But, the President may fairly say, we don't live in an originalist world (not yet, anyway), and many of the President's critics are not originalists on other matters. The nonoriginalist case for the President is much stronger here. There is closely analogous recent precedent — not only Obama's attack on Libya but also President Clinton's attack on Serbia in connection with the Kosovo conflict. Further, the speed of modern warfare and the exigencies of terrorism and potential nuclear attack may call for more independent presidential authority to engage with developing threats. As critics of originalism commonly say, the Constitution's rules were made for eighteenth-century conditions and may need to be adapted to modern conditions. Perhaps it makes sense to have a rule that limited airstrikes to accomplish narrow objectives can be made on presidential authority alone, while keeping in place the important check of congressional approval for substantial long-term military commitments.
UPDATE: At Volokh Conspiracy, Ilya Somin generally agrees, and has further thoughts.
Posted at 7:45 AM