The key event marking the start of the Vietnam War for the United States is the Gulf of Tonkin Incident, which occurred in August 1964. This incident involved alleged attacks on U.S. naval vessels by North Vietnamese forces in the Gulf of Tonkin. The U.S. government, led by President Lyndon B. Johnson, used these incidents as justification for expanding military involvement in Vietnam. Subsequently, the Gulf of Tonkin Resolution was passed by Congress, allowing the president to take military action without a formal declaration of war. This marked a significant escalation in U.S. involvement in Vietnam, leading to a full-scale conflict.