The event that marked the beginning of U.S. intervention in the Vietnam War was the Gulf of Tonkin Incident in August 1964. This incident involved reported attacks on U.S. naval vessels by North Vietnamese forces, leading Congress to pass the Gulf of Tonkin Resolution. This resolution granted President Lyndon B. Johnson broad authority to use military force in Vietnam without a formal declaration of war. This marked the escalation of American involvement, transitioning from advisory roles to direct military engagement, significantly shaping the conflict's trajectory.