Answer:
According to the Constitution, Congress has the right to declare war (Article I, paragraph 8) and the President is the commander-in-chief of the armed forces (Article II, paragraph 2). It is therefore unclear to what extent the President, as Commander-in-Chief, has the right to deploy armed forces in armed conflicts without Congress approval.
The background to the War Powers Resolution was the unclear constitutional situation regarding the use of armed forces in the Korean and Vietnam War. In the case of the Korean War, there was no formal declaration of war or Congress resolution to approve the operation. After a Vietnamese attack on an American speedboat in the Gulf of Tonkin in 1964, Congress passed a resolution that supported the use of armed forces in Vietnam. However, it was not a formal declaration of war. In 1973, the War powers resolution was passed by Congress with a two-thirds majority, after a veto by President Nixon. The aim was to ensure the future participation of the Congress in the use of armed forces.