Build-up (1960 - 1964)
In the late 1950s, a communist guerrilla force – the Vietcong – emerged in the south.
Supported and supplied from Hanoi via a network of tracks known as the Ho Chi Minh trail, its ranks were boosted by southerners frustrated with the corrupt and repressive government of their self-appointed president, Ngo Dinh Diem.
The US had been providing aid and military equipment and training to south Vietnam since 1954.
As the Vietcong grew this was increased, with US helicopters, armed personnel carriers and thousands of military advisers on the ground.
But by 1963, Diem's government was so discredited that the US did nothing to stop a coup by dissident generals.
A series of short-lived and unstable governments followed, proving no more effective against the insurgency.
The catalyst for deeper US involvement came in August 1964, when north Vietnamese torpedo boats shot at a US destroyer in the Gulf of Tonkin.
US President Lyndon Johnson used a second, and highly disputed, second clash to justify air strikes on naval bases in the north.
By the end of 1964, there were 23,000 US military advisers in Vietnam – up from 800 in the 1950s - and the Vietcong was attacking US personnel and bases directly.