As a legal matter, however, the United States' enemy is-and always was-Al-Qaeda, its allies, including the Taliban, and other jihadists who have joined the fight. Al-Qaeda was responsible for the September 11 attacks, and Congress formally authorized the use of American military force against those responsible, and anyone giving them assistance or refuge, on September 18, 2001. In American law, such authorizations are the practical equivalent of a declaration of war, although of a more limited scope, as the United States Supreme Court has recognized since Congress authorized naval hostilities against Revolutionary France in the 1790s.
At the same time, under international law, the concept of "war" or "armed conflict" has never been limited to fighting between sovereign states. There is certainly no established international norm forbidding a state's invocation of the law of armed conflict when militarily engaging a non-state actor. It is true, of course, that states have rarely declared war on non-state entities. But that formality has not been considered necessary to the existence of a legal state of war at least since the 18th century. As Emmerich de Vattel explained in his 1758 Law of Nations: "He who is attacked and makes only a defensive war, need not declare it, the state of war being sufficiently determined by the declaration of the enemy, or his open hostilities." Indeed, today it is the nature, intensity and duration of hostilities that determine application of the laws of war, rather than any formal declaration.
By that standard, the United States has been engaged in an armed conflict with Al-Qaeda since the 1990s (thus legally justifying President Clinton's 1998 strikes), when Osama bin Laden began deliberately to target American interests-including United States diplomats in Kenya and Tanzania, U.S. military personnel in Saudi Arabia and the USS Cole in Aden harbor. The September 11, 2001, attacks were certainly acts of war in terms of their nature, scope and level of violence. Indeed, they were classic examples of a pre-emptive strike designed to knock out the enemy's political, military and financial leadership. Al-Qaeda's 9/11 targets-the Pentagon, the World Trade Center and (most likely) the United States Capitol-were emphatically not selected simply to terrorize the civilian population.
The fact that Al-Qaeda is not a state, and has no legal right to make war against states, does not change this analysis. Al-Qaeda cannot legally make or declare war against the United States, but the United States is fully entitled on its part to invoke the laws of war to govern hostilities with Al-Qaeda. And it is bound only by the treaties it has ratified; the United States rejected Protocol I, and is properly considered a persistent objector to any customary international-law norms that may be forming around that instrument. Its detention of captured jihadists may not be consistent with the obligations of European Union states, but America is not an EU member or a candidate for membership.
INVOKING THE laws of war has permitted the United States both to confront Al-Qaeda and to preserve its own institutions. This was the right decision, and remains so. Given the stakes involved, the United States should continue to eschew the embrace of the European-style law-enforcement approach for dealing with the jihadi threat. At the same time, however, it must engage in an assertive advocacy of the legal and policy merits of using war's distinctive legal architecture to meet this threat-not because there is much hope of winning over European governments (barring a major escalation in the level of terrorist violence on European soil)-but to vindicate the American position and ensure its continuation as the only viable means ultimately of stopping Al-Qaeda.
In this regard, the U.S. response must go beyond "business as usual" bureaucratic policymaking, in which conflict avoidance with our allies routinely takes precedence, and legal issues are generally viewed as relatively unimportant. Whenever the Europeans accuse the United States of not being true to our common values or violating international law, we should respond with equal vigor and passion. In particular, the United States should not always be on the defensive and should be prepared to challenge the European resort to the "illiberal" law-enforcement measures. Legal and policy démarches should be issued frequently and at the highest level, with a particular emphasis on why the use of the law-enforcement paradigm is inadequate and the utilization of the laws-of-war paradigm is both legitimate and lawful.
David B. Rivkin, Jr. and Lee A. Casey are partners in the Washington, DC office of Baker Hostetler LLP, members of the UN Subcommission on the Promotion and Protection of Human Rights and have served in a variety of legal and policy positions in the Reagan and George H. W. Bush Administrations. They are writing a book on war's evolving legal architecture.
 British War Office, Manual of Military Law 240 (1914).
 See David B. Rivkin, Jr. and Lee A. Casey, "The Rocky Shoals of International Law", The National Interest, No. 62 (Winter 2000/01).
 Donald M. Nicol, The Last Centuries of Byzantium 1261-1453, 2nd ed. (New York: Cambridge University Press, 1993).