The War Powers Resolution (WPR) states that the President’s powers as Commander in Chief to introduce U.S. forces into hostilities or imminent hostilities can only be exercised pursuant to (1) a declaration of war; (2) specific statutory authority; or (3) a national emergency created by an attack on the United States or its forces.
It requires the President in every possible instance to consult with Congress before introducing American Armed Forces into hostilities or imminent hostilities unless there has been a declaration of war or other specific congressional authorization.
It also requires the President to report to Congress any introduction of forces into hostilities or imminent hostilities; into foreign territory while equipped for combat; or in numbers which substantially enlarge U.S. forces equipped for combat already in a foreign nation.
Once a report is submitted, or ‘required to be submitted’, Congress must authorize the use of force within 60-90 days or the forces must be withdrawn.
The North Atlantic Treaty provides guidance on the operations of its members. Article II of the Treaty states that its
…provisions are to be carried out by the parties “in accordance with their respective constitutional processes”, implying that NATO treaty commitments do not override U.S. constitutional provisions regarding the role of Congress in determining the extent of U.S. participation in NATO missions.
Section 8(a) of the WPR states specifically that authority to introduce U.S. forces into hostilities is not to be inferred from any treaty, ratified before or after 1973, unless implementing legislation specifically authorizes such introduction and says it is intended to constitute an authorization within the meaning of the War Powers Resolution.