The Vietnam War was a conflict between communist North Vietnam and U.S.-backed South Vietnam that ended with a ceasefire agreement in 1973.
The war had several stages: The first was military conflict between North and South Vietnam from 1945-1954 (First Indochina War). The second stage was from 1955-1975; it included two major phases:
A period of political instability, coup attempts, and reform efforts called the “Diem era”, which ended with Diem’s assassination in 1963; and
A period of renewed political stability under General Duong Van Minh (“Big Minh”), who ruled after Diem’s death until 1965.
In 1965, the U.S. began heavy bombing of North Vietnam in an attempt to force them to negotiate a peace treaty that would allow American troops to withdraw from South Vietnam without losing face. After years of fighting against both Viet Cong guerrillas and U.S. forces, Saigon finally fell on April 30th, 1975.
The United States became involved in Vietnam because it feared that if communism spread across Southeast Asia, it would threaten other countries like Thailand, Cambodia and Laos — all allies of the United States at that time. President Dwight Eisenhower believed that if Vietnam fell under communist rule, other Southeast Asian countries would follow suit and become communist themselves.