Although it was not the first time Americans had been killed in Vietnam, the Pleiku incident occurred within weeks after President Johnson’s inauguration (on January 20, 1965). Previously, American military advisors were not permitted to retaliate if fired upon.
Immediately following the Camp Holloway mortar attack (in South Vietnam), LBJ responded with Operation Flaming Dart, a
retaliatory air strike (in North Vietnam). It was the first American bombing of North Vietnam.
Before ordering the bombing run, LBJ met with his National Security advisors. (Scroll down 40% to review the official summary of those discussions.) Undersecretary of State George Ball made an interesting observation:
We need to answer the question as to why we are striking North Vietnam when the Americans were attacked by Viet Cong in South Vietnam.
Ball essentially provided the answer when he also observed:
We must make clear that the North Vietnamese and the Viet Cong are the same. We retaliate against North Vietnam because Hanoi directs the Viet Cong, supplies arms, and infiltrates men.
Some of the military advisors had their families with them in South Vietnam. Responding to concerns about the safety of those military dependents, General Wheeler [Chief of the Joint Chiefs] and Secretary of Defense Robert McNamara had a quick answer:
Get them out now.
President Johnson had decided to "go big" in Southeast Asia. The battles of Ia Drang were only months away.