The Constitution (Article I, section 8) grants Congress the power to declare war. However, it also names the President as the Commander in Chief of the armed forces (Article II, section 2.) As a result, the executive and legislative branches have often had conflicts over the power to send troops into hostilities, whether called "war" or not. For example, neither the war in Korea (1950-1953) nor the war in Vietnam (1965-1973) were ever formally declared a "war."

The War Powers Act in 1973 was enacted to promote better cooperation between the President and Congress. It states that the President must consult Congress prior to committing U.S. troops. He must report any troop commitments to Congress within 48 hours of their deployment, and must end any troop deployment if Congress has not formally declared war or given its approval by resolution within 60 days. Under some circumstances, the President may extend that period for an additional 30 days if necessary to withdraw troops safely. The Act also grants Congress the ability to pass a resolution directing the President to remove U.S. troops from foreign soil, within a stated timeframe.

Presidents have not always consulted Congress prior to sending troops abroad. Court cases brought to clarify the division of power between the two branches over troop deployment have been more advisory than definitive, urging the President and Congress to work together more cooperatively.

This area is far from settled. Overall, the President seems to have maintained the upper hand. He takes military action first and informs Congress second, which is then caught between supporting U.S. personnel en route to or en site in dangerous territory, or taking the time to debate and pass a resolution either declaring war or requiring troops to be withdrawn. Congress has declared war formally only four times: (1) the War of 1812; (2) the Spanish-American War; (3) World War I; and (4) World War II. A fifth instance, the War with Mexico, 1846-1848, is sometimes included in this list. President Polk had notified Congress on May 11, 1846 that "war exists" with Mexico, after initiating hostilities on April 25. Congress passed a joint resolution recognizing that fact two days later, and allocated resources. Two years later, it passed a resolution condemning the war as "unnecessarily and unconstitutionally begun by the President of the United States."

In recent years, troop actions in Grenada, Panama, Honduras, the Dominican Republic, Lebanon, Iran, Iraq, the Persian Gulf, Zaire, and the Sudan were all undertaken at the initiation of the President, without prior vote of Congress.