The correct answer is "The Gulf of Tonkin Resolution".
The Gulf of Tonkin Resolution was enacted on August 10, 1964. It was the initial action that the US government took regarding the Vietnam War. After the "Gulf of Tonkin Incident", which saw the USS Maddox engage into combat with 3 North Vietnamese torpedo boats on August 2, 1964, President Lyndon Johnson was granted the authorization by US Congress to formally declare war on North Vietnam.