The War Powers Resolution is a federal law that states that the president must obtain Congress's approval to commit American forces to combat. The president must notify Congress within 48 hours of deploying forces, and troops must be withdrawn in 60 days without a declaration of war or similar resolution. The only exception is in response to a national emergency caused by an attack on the United States.
The War Powers Resolution was passed in 1973 in response to the military conflicts in Korea and Vietnam. In both cases, the president at the time deployed forces for an extended war without gaining approval from Congress, bypassing one of the legislative branch's powers enshrined in the Constitution. In particular, President Nixon's secret bombings of Cambodia triggered this legislative response, since he had neglected even to inform Congress that forces had been deployed for that purpose. Nixon vetoed the War Powers Resolution, but his veto was overturned by Congress, and the bill became law.
Even with the War Powers Resolution in force, presidents have managed to find ways around its restrictions. President Clinton kept American forces deployed in Kosovo beyond the deadline by arguing that Congress's funding of military operations was a tacit authorization for the operation. In 2011, President Obama kept American forces involved in Libya by arguing that NATO had taken over responsibility for operations there. In response, Congress has considered new and more restrictive legislation to replace the law.