War Powers Resolution
In passing the War Powers Resolution on November 7, 1973, the Congress of the United States attempted to balance law, the changing world environment, and politics. It was passed over the veto of President Richard Nixon.
Formally, the Constitution of the United States gives Congress (Article I, section 8) the authority to declaration of war, with the President separately designated (Article II, section 2) as Commander-in-Chief of military forces whose funding is under congressional control. In the eighteenth and nineteenth centuries, it was quite possible to Congress to deliberate questions of war. With the increasing frequency of surprise attacks in the twentieth century, it was understood forces needed to be authorized to act in self-defense, but, even after the Battle of Pearl Harbor, a declaration of war was still used.
With the introduction of nuclear weapons, situations clearly came into being where there could be no time for Congressional action, and it was accepted that a President might have to make a quick major action. In some cases, such as the Korean War, U.S. action could quickly tie to international actions.
Things changed, however, with the Gulf of Tonkin incident, where, to repel what was a relatively small attack, which was no direct threat to the United States, the Congress passed the Gulf of Tonkin Resolution. It became the authorization for a very major war. The War Powers Resolution was, in large part, a response to what was seen as unexpected consequences of the Gulf of Tonkin Resolution. The purely military Total Force Concept, while having complex motivations, was another response to overcommitment.
Consultation and reporting
Section 1542 of the Resolution recognizes that there may be emergent situations where the President may have to initiate hostilities under his authority as Commander-in-Chief, but he is required to"consult with Congress before introducing United States Armed Forces into hostilities or into situations where imminent involvement in hostilities is clearly indicated by the circumstances, and after every such introduction shall consult regularly with the Congress until United States Armed Forces are no longer engaged in hostilities or have been removed from such situations.
The House report gave some framework for when authorization was needed:
While the word "substantially" designates a flexible criterion, it is possible to arrive at a common-sense understanding of the numbers involved. A 100% increase in numbers of Marine guards at an embassy -- say from 5 to 10 -- clearly would not be an occasion for a report. A thousand additional men sent to Europe under present circumstances does not significantly enlarge the total U.S. troop strength of about 300,000 already there. However, the dispatch of 1,000 men to Guantanamo Bay, Cuba, which now has a complement of 4,000 would mean an increase of 25%, which is substantial. Under this circumstance, President Kennedy would have been required to report to Congress in 1962 when he raised the number of U.S. military advisers in Vietnam from 700 to 16,000.
Section 1544 essentially establishes a sixty-day timer for the President either to get further authorization or withdraw forces. Reporting requirements to support this rule are in Section 1543, which require,
- The President shll report to the Speaker of the House of Representatives and to the President pro tempore of the Senate on the same calendar day; they will transfer the reports to the Committees on Foreign Affairs of the House of Representatives and to the Committee on Foreign Relations of the Senate for appropriate action.
- If Congress is in adjournment or recess, the Speaker and President pro tempore, "if they deem it advisable (or if petitioned by at least 30 percent of the membership of their respective Houses) shall jointly request the President to convene Congress" to consider the matter.
Use of force must be terminated unless:
- Congress has declared war or has enacted a specific authorization for such use of United States Armed Forces (e.g., Authorization for the Use of Military Force,
- has extended by law such sixty-day period, or
- is physically unable to meet as a result of an armed attack upon the United States, which may be extended for another sixty days.
The Resolution intended,
... to fulfill the intent of the framers of the Constitution of the United States and insure that the collective judgment 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.
From 1975 through 2003, Presidents have submitted 111 reports as the result of the War Powers Resolution, but only one, the 1975 Mayaguez seizure, mentioned the time limit, and in this case the military action was completed and U.S. armed forces had disengaged from the area of conflict when the report was made.
On June 7, 1995, the House defeated, by a vote of 217-201, an amendment to repeal the central features of the War Powers Resolution that have been deemed unconstitutional by every President since the law's enactment in 1973.
Individual legislators, Rep. Tom Campbell in 1999, sought unsuccessfully to challenge the use of U.S. forces without Congressional authorization, as in Yugoslavia.John Yoo wrote that President Bill Clinton exceeded the sixty-day authority of the Resolution when he joined in NATO operations in Kosovo without authorization by both Houses. While the Senate passed a concurrent resolution to "conduct military air operations and missile strikes in cooperation with our NATO allies against the Federal Republic of Yugoslavia (Serbia and Montenegro),"  the House refused to authorize action. After Clinton's public announcement of the commencement of operations, the House declared it supported the troops despite the "deep reservations" of some Members. Yoo wrote that Clinton followed the examples of Presidents Reagan and George H. W. Bush, and made it clear that while he appreciated the support, he did not need its authority. He said he was acing "consistent with" rather than "pursuant to" the War Powers Resolution.
On April 28, the House rejected a joint resolution declaring war on Yugoslavia, defeated (on a tie) a Senate resolution authorizing the use of force, and then defeated a resolution that would have ordered the President to remove U.S. troops from Yugoslavia.
Nevertheless, on May 20, Congress doubled the Administration's emergency funding request but still did not authorize the war.
Congress has never authorized the insertion of American troops, who remain in Kosovo to this day. Congress, however, agreed to provide supplementary appropriations for a long-term military presence in Kosovo, and in the years since has continued to appropriate funds for a long-term military presence in Kosovo...Congress could have stopped the war, if it possessed the political will, merely by refusing to appropriate the funds to keep the military operations going.
One might respond that it is unreasonable to expect Congress to use its appropriations to cut off troops in the field. (John Ely and Harold Koh)...claim that requiring congressmen to cut off funds is unrealistic given the pressure to show support for troops in the field...We should not, however, mistake a failure of political will for a violation of the Constitution. 
On 9 October 2001, President George W. Bush reported to Congress that he was taking action against threats in Afghanistan, "consistent with" the Resolution. This was supported by Senate Joint Resolution 23. Both the Executive and Legislative branches held to their continuing positions. 
- United States Congress, War Powers Resolution, Cornell University Law School, 50 USC 1541-1548
- Richard F. Grimmett (March 11, 2004), The War Powers Resolution: After Thirty Years, Congressional Research Service, CRS Order Code RL32267
- U.S. Congress. H.Rept. 93-287, p. 8., cited by Grimmett
- Senate Concurrent Resolution 21, 106th Congress, 1999
- H.R. Resolution 130, 106th Congress, 1999
- John Yoo (2005), The Powers of War and Peace: the Constitution and Foreign Affairs since 9/11, University of Chicago, ISBN 0226960315, pp. 157-159
- Richard F. Grimmett, Gerald M. Perkins (2005), The War Powers Resolution after thirty years, Nova, ISBN 9781594547201, p. 55