In the Gulf of Tonkin Resolution, Congress gave President Johnson the power to "take all necessary measures to repel any armed attack against the forces of the United States and to prevent further aggression.", with the purpose of promoting international peace and security and support the defense of U.S. naval vessels in international waters from repeated and calculated attacks from Communist naval units in Vietnam.
The War Powers Act (1973) was issued by Congress following Executive decisions made during the Korean and Vietnam Wars. The War Powers Act defines and limits the powers of the president to command the armed forces. *
With the War Powers Act (1973) the president had to get a resolution from congress to authorize war and if the resolution wasn’t passed, armed forces had to withdraw from combat within sixty days.