The Constitution of the United States stipulates that Congress and the President share war powers. The Congress, under Article I, Section 8, has the power to declare war, raise and support armies, provide and maintain a navy, make rules for the government and regulation of the land and naval forces. The President is Commander in Chief, according to Article II, Section 2. He can use armed forces to repel attacks against the country. However, there is no common consent that the President has power to send forces into hostile situations abroad without a declaration of war or other congressional authorization. The President "shall from time to time give to the Congress Information of the State of the Union," according to the Constitution.
On November 7, 1973, Congress passed the War Powers Resolution, despite the veto of President Richard M Nixon. After the Korean War (1953-1956), concerns about the presidential use of armed forces without an approval of Congress grew. During America's involvement in the Vietnam War during the 1960s, Congress tried to find a way to assert authority to decide when the United States could utilize armed forces abroad in hostilities or potential hostilities. The resolution set procedures for legislative and executive branches to share in decisions that might get the U.S. involved in war.
According to the War Powers Resolution, its main purpose is to "insure that the collective judgement of both the Congress and the President will apply to the introduction of United States Armed Forces into hostilities, or into situations where imminent involvement in hostilities is clearly indicated by the circumstances, and to the continued use of such forces in hostilities or in such situations."
The President has to consult with Congress "in every possible instance," before involving the armed forces into situations of hostilities and imminent hostilities. He also has to report to Congress whenever he introduces armed forces abroad. In the absence of a declaration of war or congressional authorization, he has to report within 48 hours.
Hostilities, according to the War Powers Resolution, also includes a state of confrontation in which no shots have been fired but where there is a clear and present danger of armed conflict. Imminent hostilities, in turn, describe a situation in which there is a clear potential either for such a state of confrontation or for actual armed conflict.
The President is obliged to cease the use of armed forces after 60 days unless the Congress has declared war or approved the action. The other two hypotheses under which the President could continue with the use of armed forces are if Congress has extended the period by law or if there is an armed attack on the United States. He can extend the 60 days for additional 30 days if there is "unavoidable military necessity respecting the safety of United States Armed Forces," which will require their continued use. In any case, the President will have to remove the forces at any time if Congress directs it by concurrent resolution.
Presidents after Nixon have argued that the provision, permitting Congress to direct the withdrawal of troops by concurrent resolution, is unconstitutional. It limits the President's authority as Commander in Chief. The main problem between the legislative and executive branch comes from different interpretations of the war powers. Congress believes that the framers of the Constitution gave it the power to declare war, meaning the final decision whether or not to enter a war, and did not grant the President the power to commit armed forces to war. The Commander in Chief function allows him to lead the army and to defend the nation but not to take the decision to start war, according to the Congress' interpretation.
The President considers that the Constitution has empowered him as Commander in Chief to wage war. Therefore, the Resolution imposes restrictions upon the authority of the President which could harm the interests of the United States and "attempt to take away, by a mere legislative act, authorities which the President has properly exercised under the Constitution for almost 200 years," according to President Nixon.