The War Powers Act was a law passed by Congress because it would limit the power of the executive branch.
This act was passed after the Vietnam War, in which Congress gave the president almost unlimited power to put troops wherever he saw fit. This was part of the Golf of Tonkin Resolution. This did not end well, as the US involvement in Vietnam was a failure.
To prevent this from happening again, the War Powers Act was passed. This made it so that the president can only put troops in a country if the United States security was at stake. The president can only put troops there for 60 days. After that, he/she needs Congress to approve troops being there.
