Vietnam War: The National Security Agency falsely claimed that a U.S. Navy destroyer was attacked a second time by North Vietnamese vessels in the Gulf of Tonkin, leading Congress to authorize the use of military force.

The Vietnam War was an armed conflict in Vietnam, Laos, and Cambodia fought between North Vietnam and South Vietnam and their allies. North Vietnam was supported by the Soviet Union and China, while South Vietnam was supported by the United States and other anti-communist nations. The conflict was the second of the Indochina wars and a proxy war of the Cold War between the Soviet Union and US. The Vietnam War was one of the postcolonial wars of national liberation, a theater in the Cold War, and a civil war, with civil warfare a defining feature from the outset. Direct US military involvement escalated from 1965 until its withdrawal in 1973. The fighting spilled into the Laotian and Cambodian Civil Wars, which ended with all three countries becoming communist in 1975.

The National Security Agency (NSA) is an intelligence agency of the United States Department of Defense, under the authority of the director of national intelligence (DNI). The NSA is responsible for global monitoring, collection, and processing of information and data for global intelligence and counterintelligence purposes, specializing in a discipline known as signals intelligence (SIGINT). The NSA is also tasked with the protection of U.S. communications networks and information systems. The NSA relies on a variety of measures to accomplish its mission, the majority of which are clandestine. The NSA has roughly 32,000 employees.

The Gulf of Tonkin incident was an international confrontation which led to the United States engaging more directly in the Vietnam War. It consisted of a confrontation on 2 August 1964, when US forces were carrying out covert amphibious operations close to North Vietnamese territorial waters, which triggered a response from North Vietnamese forces. The US government falsely claimed that a second incident occurred on 4 August, between North Vietnamese and United States ships in the waters of the Gulf of Tonkin. Originally, US military claims blamed North Vietnam for the confrontation and the ostensible, but in fact imaginary, incident on 4 August. Later investigation revealed that the second attack never happened. The National Security Agency, an agency of the US Defense Department, had deliberately skewed intelligence to create the impression that an attack had been carried out.