Answer: The War Powers Resolution
Detail:
Passed in 1973 over the veto of President Richard Nixon, the War Powers Resolution (its official name) blocks presidents from continuing the pursuance of a war without Congress's approval. This was a way of curtailing the authority of presidents to increase involvement in the war, which had begun with the Gulf of Tonkin Resolution during the Johnson presidency.
The War Powers Resolution requires the President of the United States to notify Congress within 48 hours of committing armed forces to any military action. Armed forces cannot remain deployed for more than 60 days (plus a 30-day withdrawal period), unless Congress authorizes the use of military force or enacts a declaration of war.
In practice, since its passage, the War Powers Resolution has at times been sidestepped by presidents. The US Constitution leaves some tension in place between the legislative and executive branch when it comes to the country's involvement in war. Article I of the Constitution gives Congress the power to declare war. But Article II names the President the Commander-in-Chief, and presidents frequently have understood that role as containing the authority to deploy US forces without first getting congressional approval. The War Powers Resolution has been an ongoing point of controversy, as US forces have been sent into all sorts of conflict zones in the 21st century without formal declarations of war.