Sometimes referred to as the War Powers Clause, the United States Constitution, Article One, Section 8, Clause 11, vests in the Congress the exclusive power to declare war.

Six wars have been declared in American history: the Barbary Coast War, the War of 1812, the Mexican-American War, the Spanish-American War, World War I and World War II. Some historians argue that the legal doctrines and legislation passed during the operations against Pancho Villa constitute a seventh declaration of war.

However, beginning with the Korean War, American presidents have not sought formal declarations of war, instead maintaining that they have the constitutional authority, as commander in chief (Article Two, Section Two) to use the military for "police actions."

In 1973, Congress passed the War Powers Resolution, which requires the president to obtain either a declaration of war or a resolution authorizing the use of force from Congress within 60 days of initiating hostilities. Its constitutionality has never been tested, as Congress has always passed the required authorization when requested by the president.

Some legal scholars maintain that all military action taken without a Congressional declaration of war (regardless of the War Powers Resolution) is unconstitutional; the Supreme Court has never ruled directly on the matter.