The War Powers Resolution (also known as the War Powers Resolution of 1973 or the War Powers Act) (50 U.S.C. 1541–1548) is a federal law intended to check the president's power to commit the United States to an armed conflict without the consent of the U.S. Congress. The Resolution was adopted in the form of a United States Congress joint resolution. It provides that the U.S. President can send U.S. Armed Forces into action abroad only by declaration of war by Congress, "statutory authorization," or in case of "a national emergency created by attack upon the United States, its territories or possessions, or its armed forces."
The War Powers Resolution requires the President to notify Congress within 48 hours of committing armed forces to military action and forbids armed forces from remaining for more than 60 days, with a further 30-day withdrawal period, without a Congressional authorization for use of military force (AUMF) or a declaration of war by the United States. The resolution was passed by two-thirds of Congress, overriding the veto of the bill from President Nixon.
It has been alleged that the War Powers Resolution has been violated in the past – for example, by President Bill Clinton in 1999, during the bombing campaign in Kosovo. Congress has disapproved all such incidents, but none has resulted in any successful legal actions being taken against the president for alleged violations.
This summary is from Wikipedia.
The summary below was written by the Congressional Research Service, which is a nonpartisan division of the Library of Congress.
(LATEST SUMMARY) War Power Resolution - Declares that it is the purpose of this Act to fulfill the intent of the framers of the Constitution of the United States and insure that the collective judgment of both the Congress and the President will apply to the introduction of the Armed Forces of the United States in hostilities, or in situations where imminent involvement in hostilities is clearly indicated by the circumstances, and to the continued use of such forces in hostilities. Requires that the President shall in every possible instance consult with Congress before introducing United States Armed Forces into hostilities or into situations where imminent involvement is clearly indicated by the circumstances. Provides that in the absence of a declaration of war by the Congress, in any case in which the Armed Forces of the United States are introduced in hostilities, or in situations where imminent involvement in hostilities is clearly indicated by the circumstances, such use of the Armed Forces of the United States in hostilities pursuant to this Act shall be reported within 48 hours in writing by the President to the Speaker of the House of Representatives and the President pro tempore of the Senate, together with a full account of the circumstances under which such hostilities were initiated, the estimated scope and duration of such hostilities, and the constitutional and legislative authority under which the introduction of hostilities took place. Provides that nothing in this Act is intended to alter the provisions of existing treaties. Sets forth the criteria for Congressional consideration of joint resolutions and concurrent resolutions introduced pursuant to this Act. Provides that this Act shall take effect on the date of its enactment.