Gulf of Tonkin incident
In 1964, the USS Madox fired three "warning" shots at three North Vietnamese Navy torpedo boats. The torpedo boats returned fire, a mix of torpedoes and machine gun fire being used. The USS Maddox fired more than 280 shells in a sea battle. As a result the three torpedo boats were damaged alongside one US aircraft.
The NSA (National Security Agency) claimed that a second such incident had happened two days after the first image (August 4th) but evidence supplied turned out to be false radar images rather than an actual incident. Later evidence has proved overwhelmingly that such an event didn't happen.
Following this the Gulf of Tokin Resolution was passed in the US Congress. This gave President Lyndon B. Johnson authorisation to provide assistance to any country in Southeast Asia who was at risk of "communist aggression." Johnson made use of the resolution to commence an open war against North Vietnam, with a deployment of US forces.
The US government initially denied that warning shots had been fired at the torpedo boats.