The crisis precipitated by a Syrian attack on civilians using chemical weapons, in defiance of President Barack Obama's promise that there would be consequences for the regime if such an attack were to occur, provides a timely occasion to clear up some of the confusions that have accreted in the last 50 years around the question of the president's constitutional authority to use armed force.
The seeds of this confusion were sown in the 1960s, when President Lyndon B. Johnson sought and won the overwhelming approval of both houses of Congress for the Gulf of Tonkin Resolution supporting the administration's deepening involvement in Vietnam. Johnson did not seek that resolution on constitutional grounds, but rather because he believed President Harry Truman had been mistaken, on practical political grounds, when he relied on the U.N. Charter and Security Council authority to commit troops to the U.N. expeditionary force in Korea. As the U.S. intervention in Vietnam became more controversial, the anti-war movement and their repentant allies in Congress who had supported the resolution attempted to vitiate its legal authority by claiming that they had been swindled, and had never anticipated -- despite the clear, open-ended language of the text of the resolution -- the war that resulted.
This was given the imprimatur of history by the claim, so faithfully repeated in countless books and articles since, that the entire Tonkin incident had been concocted by the Johnson administration in order to win approval for the resolution. Although there subsequently emerged doubt as to whether the second North Vietnamese attack on a U.S. destroyer in fact occurred or was erroneously reported by the local commander (whose men mistakenly took sonar reverberations for a torpedo), there is no doubt -- and there never has been any basis for doubting -- that the first attack did indeed take place and that President Johnson was just as startled by this as anyone else.
Nevertheless, this studied warping of history had an ironic effect: it persuaded presidents and their advisors that there was little to be gained by seeking congressional authority. As Johnson himself put it, "I thought if they were with me on the take-off they'd be with me until we landed. I forgot they had parachutes."
"Case law" is the phrase lawyers use to describe the judicial precedents that create a substantive body of law; in constitutional law, these precedents are the bases for doctrinal argument. But there are also doctrinal arguments as to constitutional subjects that are not justiciable, i.e., that cannot be decided by courts. Some examples of such subjects include the grounds for impeachment, the permissible bases on which the rejection of presidential nominees can be made, the question of whether the executive may by treaty purchase land for the United States (like the Louisiana Purchase) in advance of a congressional appropriation, and, of course, the constitutional requirements the executive must satisfy in order to go to war. The difference between this sort of case law and the opinions collected in the Supreme Court Reports is that the precedents are written in history -- not in judicial decisions.
There is ample precedent in recent history -- and with precedent, the more recent, the more authoritative -- to support the view that the president does not need a joint resolution of Congress, much less a declaration of war, in order to initiate hostilities on a valid constitutional basis. The rescue of the Mayaguez (Ford), the Iran hostage rescue attempt (Carter), the invasion of Grenada (Reagan), the intervention in Lebanon (Reagan), the invasion of Panama (Bush), the air attacks on Serbia (Clinton), and the cruise missile attacks on Afghanistan and Sudan (Clinton) are all recent examples.
Beyond the presidential case law, the history and text of the Constitution support the president's authority to take unilateral military action, though that may come as a surprise to many people. The Constitutional Convention of 1787 removed the power to make war from both Article I and Article II, that is, from the executive and legislative branches, giving the president the power to command forces, and Congress the power to provide funds for those forces. The power to declare war was never considered a precondition for entering hostilities but rather, as the Supreme Court observed in 1800 in Bas v. Tingy, a matter of "perfecting" an otherwise limited war and thus conferring certain rights under international law vis-à-vis neutrals, and other parties. This explains why our first war -- the so-called quasi-war against France in the John Adams administration, when many constitutional framers were alive -- was not preceded by a declaration of war. Nor, by the way, was President Thomas Jefferson's expedition against the Barbary pirates, which also relied on statutory authority.