The Gulf of Tonkin Resolution, passed by the U.S. Congress in 1964, was significant as it provided the legal basis for the escalation of U.S. military involvement in the Vietnam War. Following alleged attacks on U.S. naval vessels by North Vietnamese forces, this resolution granted President Lyndon B. Johnson broad authority to take military action without a formal declaration of war. Its significance lies in expanding American engagement in Vietnam, leading to increased troop deployments and deepening U.S. involvement in a conflict that resulted in significant human and political costs.