It is an act of congress passed after the Vietnam War, over President Nixon
On this day in 1973, President Richard Nixon vetoed the War Powers Resolution, which had cleared the House 238-123, branding the bill as “unconstitutional and dangerous.” In November of that year, Congress overrode his veto, reflecting its deep hatred to the Vietnam War.
http://en.wikipedia.org/wiki/War_Powers_Act_of_1941