One of the standard bearers of the Fact Free Parade that is High School History (oops - Social Studies) is the fact that the Gulf of Tonkin Incident caused the outbreak of the Vietnam War. There have been rumblings from the beginning that it was a hoax, that there was no attack, but these voices have been shouted down - yelled at and called "conspiracy theorists", and not to be discussed in serious venues with serious people.
With the advent of the internet, it turns out that the Conspiracy Kooks were right after all.
The two 'events', on August 2nd 1964 and August 4th, 1964 were, and are by many, seen as the belligerent acts by North Vietnam that warranted an escalation of troops, and eventually a war. The August 2nd event turns out to have been American boats firing first, initiating conflict. The August 4th incident never happened - it was created out of whole cloth. Looking back, we see the repeated pattern of an 'event' used to justify war - an event so flimsy that it is baffling in hindsight to see that people bought it. Not everyone did, but those people were maligned by the establishment, at great cost to their careers. 58,000 American deaths, and possibly millions of Vietnamese deaths rest on a 'cause' that, in effect, never happened.
"It is not simply that there is a different story as to what happened; it is that no attack happened that night. [...] In truth, Hanoi's navy was engaged in nothing that night but the salvage of two of the boats damaged on August 2."
EDIT - Here's Jesse Ventura explaining the above phenomena to ratings challenged Piers Morgan. The fun begins around 7:00.