Why did the Vietnam War cause Congress to pass the War Powers Act?

The United States had been involved in long and difficult
conflicts in Korea and Vietnam without a Congressional Declaration of War. Under the
Constitution, only Congress can declare war, and the President acts as
commander-in-chief of the armed forces. Congress also has the authority under the
Constitution to maintain an army and navy. A large number of congressmen became
concerned that allowing the President to engage in undeclared war on his own authority
usurped the authority of Congress. There was the fear that presidential action was
gradually eroding the intent of the framers of the Constitution to gtive Congress the
ultimate authority on committing U.S. forces to combat. The situation was perhaps
aggravated because the Vietnam conflict was much longer and more involved than anyone
had previously expected. This led Congress to pass the War Powers Resolution which was
vetoed by President Richard Nixon. Both Houses of Congress overrode the veto and the
Resolution became law.