Declarations of War and Authorizations for the Use of Military Force: Historical Background and Legal Implications. Jennifer K. Elsea, Legislative Attorney. Matthew C. Weed,  Analyst in Foreign Policy Legislation. April 18, 2014

“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 enacted authorizations for the use of force rather than formal declarations of war. Such measures have generally 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. 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 enemy property, and the apprehension of enemy aliens. While a formal declaration was once deemed a necessary legal prerequisite to war and was thought to terminate diplomatic and commercial relations and most treaties between the combatants, declarations have fallen into disuse since World War II. The laws of war, such as the Hague and Geneva Conventions, apply to circumstances of armed conflict whether or not a formal declaration or authorization was issued. 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, although the executive branch has argued, with varying success, that the authorization to use force in response to the terrorist attacks of 2001 provided a statutory exception to certain statutory prohibitions.”