After World War Two a civil war was fought in Vietnam between the communists and the old French rulers and their supporters. Eventually, the French were forced to leave.
It was agreed the country would be split along the 17th Parallel until elections could be held.
The country remained divided:
The Vietminh wanted to unite the country under communist leader Ho Chi Minh. Many of the South Vietnamese people supported Ho Chi Minh as they were unhappy with Ngo Dinh Diem.
War broke out between the North and South. From 1958 onwards the South came under increasing attacks from communists in South Vietnam itself.
This was the belief that if one country fell to communism, it was likely that the neighbouring one would also fall.
The USA was afraid that communism would spread to South Vietnam. They decided to send money, supplies and military advisers to help the South Vietnamese Government.
Diem was a corrupt leader who refused to give peasants land. He did not like Buddhism and treated the majority Buddhist population badly. As a result, much of South Vietnam’s population was rebelling against him. The peasants wanted communism and supported the Vietminh.
In 1963 Diem’s Government was overthrown. After this, there was no strong capitalist government in the South.
The North Vietnamese attacked the US Navy in the Gulf of Tonkin. This incident gave the USA the excuse it needed to escalate the war. The first major contingent of US Marines arrived in 1965.