For completeness, the "Gulf of Tonkin" incident was two alleged incidents. One of them probably happened and the other almost certainly didn't (admitted by Robert McNamara years later.) The first incident, the one that probably happened, was insufficient casus belli for LBJ and then conveniently for the war hawks the second incident, which never actually happened, sealed the deal two days later.

McNamara's tacit admission: https://www.youtube.com/watch?v=HODxnUrFX6k