Declarations of War and Authorizations for the Use of Military Force: Historical Background and Legal Implications [Updated January 14, 2003] [open pdf - 435KB]
"From the Washington Administration to the present, Congress and the President have enacted eleven 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."
CRS Report for Congress, RL31133
Federation American Scientist: http://www.fas.org/