Newly-released US documents suggest the US escalated the war in Vietnam based on skewed intelligence.
A US ship was attacked on 2 August 1964. But was there a second?
The documents cast doubt on the existence of an attack on a US warship by the North Vietnamese in the Gulf of Tonkin on 4 August 1964.
The incident prompted President Lyndon Johnson to ask Congress, in effect, to declare war on Vietnam.
The revelations, released by the National Security Agency, were written by its own historian in 2001.
Robert Hanyok declares his review of all the intelligence shows beyond doubt that "no attack happened that night". The USS Maddox had been attacked two days earlier.
He claims errors were made in the translation of the intercepted signals from the North Vietnamese, and officials gave too much weight to flimsy evidence.
But he clears President Johnson and his ministers of any blame. They were only shown intelligence supporting the claim of an attack, not a wealth of contradictory material, he says.
Instead, he blames the intelligence-gathers. "They walked alone in their counsels," he wrote.
Three days later, President Johnson asked Congress to empower him to take "all necessary steps" in the region, opening the way for a war that resulted in the deaths of 58,000 Americans and three million Vietnamese.
The US government is said to have fought the declassification of the documents over fears of comparisons with the handling of Iraq, says the BBC's defence and security correspondent Rob Watson.