The event that directly led to the start of the Vietnam War was the Gulf of Tonkin Incident in August 1964. This incident involved alleged attacks on U.S. naval vessels by North Vietnamese forces in the Gulf of Tonkin. In response to these events, the U.S. Congress passed the Gulf of Tonkin Resolution, granting President Lyndon B. Johnson the authority to escalate military involvement in Vietnam. This marked the beginning of a significant increase in American troop deployments and military engagement, ultimately leading to full-scale involvement in the conflict that lasted until 1975.