From the Washington Administration to the present, Congress and the President have enacted 11 separate formal declarations of war against foreign nations in five different wars. Each declaration has been preceded by a presidential request either in writing or in person before a joint session of Congress. The reasons cited in justification for the requests have included armed attacks on United States territory or its citizens and threats to United States rights or interests as a sovereign nation.

Congress and the President have also on a number of occasions enacted authorizations for the use of force instead of declarations of war. Most commonly, such measures have authorized the use of force against either a named country or unnamed hostile nations in a given region. In most cases, the President has requested the authority, but Congress has sometimes given the President less than what he asked for. In contrast to the declarations of war, not all authorizations for the use of force have resulted in actual combat. Both declarations and authorizations require the signature of the President in order to become law.

In contrast to an authorization, a declaration of war in itself creates a state of war under international law and legitimates the killing of enemy combatants, the seizure of its property, and the apprehension of enemy aliens. At one time, a declaration was deemed a necessary legal prerequisite to a war and was also thought to terminate diplomatic and commercial relations and most treaties between the combatants. In the modern era, the international legal consequences of declarations have become less determinate; in fact, declarations have rarely been issued since World War II. Perhaps most important, neither a declaration nor an authorization is necessary to trigger application of the laws of war, such as the Hague and Geneva Conventions; for that, the fact of armed conflict is the controlling circumstance.

With respect to domestic law, a declaration of war automatically triggers many standby statutory authorities conferring special powers on the President with respect to the military, foreign trade, transportation, communications, manufacturing, alien enemies, etc. In contrast, no standby authorities appear to be triggered automatically by an authorization for the use of force. Most standby authorities do not require a declaration of war to be actualized but can be triggered by a declaration of national emergency or simply by the existence of a state of war. Declarations of war and authorizations for the use of force waive the time limitations otherwise applicable to the use of force imposed by the War Powers Resolution.

This report provides historical background on the enactment of declarations of war and authorizations for the use of force and analyzes their legal effects under international and domestic law. It also sets forth their texts in two appendices. Because the statutes that confer standby authority on the President and the executive branch potentially play such a large role in an armed conflict to which the United States is a party, the report includes an extensive listing and summary of the statutes that are triggered by a declaration of war, a declaration of national emergency, and/or the existence of a state of war. The report concludes with a summary of the congressional procedures applicable to the enactment of a declaration of war or authorization for the use of force and to measures under the War Powers Resolution. The report will be updated as circumstances warrant....