The lead up to the Vietnam War included an incident during which North Vietnamese torpedo boats supposedly attacked U.S. ships in the Gulf of Tonkin, Vietnam. SIGINT (signals intelligence, which is derived from electronic signals sent by foreign targets, such as communications systems, radars, and weapons systems) from the events shows that there was likely an attack on the USS Maddox on August 2, 1964, but that a second attack by North Vietnamese torpedo boats on U.S. ships on August 4, 1964 did not occur, despite the government's titles to the contrary.
The events, particularly the second attack, prompted Congress to pass a resolution that granted President Lyndon Johnson freedom to intervene in Vietnam, which ultimately led to the United States going to war.
The National Security Agency declassified related documents in 2005 and admitted that there was no second attack on U.S. ships in Tonkin on August 4, 1964.