Congress passed the War Powers Act in 1973 to limit the power of the U.S. president to send troops into combat without congressional authorization. The act stipulates that the president must inform Congress of commitment of troops abroad within 48 hours, withdraw troops within 60 days and complete the withdrawal within a 30-day extension period unless Congress approves the deployment.
According to Article 1, Section 8 of the U.S. Constitution, only Congress has the power to declare war. The War Powers Act was introduced during the ongoing conflict in Vietnam, which Presidents Kennedy, Johnson and Nixon had gradually escalated based on the Gulf of Tonkin Resolution without further consulting Congress. Specifically, President Nixon had been secretly bombing Cambodia without informing Congress. The act was passed by both the House of Representatives and the Senate, but President Nixon vetoed it. A subsequent congressional vote acquired the two-thirds majority necessary to override the veto.
Since its inception, the War Powers act has been controversial, and no U.S. president has specifically invoked it before sending troops into combat. Congress cited the act to limit military engagement in Somalia in 1994, in former Yugoslavia in 1999 and in Libya in 2011. Presidents asked for and received Congressional authorization for combat operations under the terms of the act in Lebanon from 1982 to 1983 and in Iraq in 1991, although they did not mention the act itself.