According to the Constitution, the power to declare war rests with Congress. Article 1, Section 8, Clause 11 states: "Congress shall have the power to declare war..."
The War Powers Resolution of 1973 was adopted as a joint resolution by the House of Representatives and the Senate in an attempt to establish a procedure requiring the president to obtain Congressional approval before sending United States military forces into action in a foreign country.
According to the requirements of the Resolution, the president had to inform Congress within forty-eight hours of committing military forces into action; it also limited the length of such a military involvement to sixty days, with an additional thirty days allowed for withdrawal of forces, unless Congress authorized further use of force or formally declared war. The purpose of the Resolution was to give the citizens of the United States, as represented by their Congressional delegates, the power to approve committing personnel, equipment, and funds to foreign military actions.
The War Powers Resolution was vetoed by President Richard Nixon, but the veto was overridden by Congress. Since 1973, presidents have committed troops to foreign military involvements without following all limitations contained in the War Powers Resolution, contending that the Resolution is unconstitutional