Decades of civil conflict in Vietnam paved the way for the entanglement of the United States in the war in Indochina. Soon after the end of World War II, the guerrilla forces which had resisted Japan in the north turned their energies against the colonial power of France, the current occupying force in Vietnam. The Democratic Republic of Vietnam was established in Hanoi with Ho Chi Minh as president. In My, 1954, after years of escalating military conflict, the French and Ho's communist forces signed an agreement calling for an armistice and the temporary division of the country with French authority consolidated around Saigon in the southern half of the country. In 1963, southern military leaders, with the support of the U.S., overthrew the government of Ngo Dinh Diem.

The new military government that took Diem's place was weak, however, and by late 1964, South Vietnam was in...