Vietnam became a French colony in 1877 with the founding of French Indochina, which included Tonkin, Annam, Cochin China and Cambodia. (Laos was added in 1893.) The French lost control of their colony briefly during World War II when Japanese troops occupied Vietnam.

As Japan and France fought over Vietnam, an independence movement was forming under Ho Chi Minh, a revolutionary leader inspired by Lenin’s Bolshevik Revolution. He established the League for the Independence of Vietnam, better known as the Viet Minh, in May of 1941.

Ho Chi Minh declared Vietnam’s independence from France on September 2, 1945, just hours after Japan’s surrender in World War II. When the French rejected his plan, the Viet Minh resorted to guerilla warfare to fight for an independent Vietnam.

The conflict between the French and the Viet Minh came to a head at the decisive Battle of Dien Bien Phu, when, after a four-month siege, the French lost to the Viet Minh under commander Vo Nguyen Giap, marking the end of French rule in Vietnam. The question of who would rule Vietnam and how drew the interest of world superpowers, who watched the situation in Vietnam with growing unease.

The Geneva Accords were signed in July of 1954 and split Vietnam at the 17th parallel. North Vietnam would be ruled by Ho Chi Minh’s communist government and South Vietnam would be led by emperor Bao Dai. An election was scheduled in two years’ time to unify Vietnam, but the U.S., fearful that a national election would lead to communist rule, ensured it never took place.

“The ‘temporary’ division of the country at the seventeenth parallel into two ideologically-opposed states meant that the civil conflict in Vietnam would collide full-scale with the East-West rivalry,” says Lien-Hang T. Nguyen, Dorothy Borg Associate Professor in the History of the United States and East Asia at Columbia University.

Vietnam was divided during the Cold War when tensions between the U.S. and The Soviet Union were at an all-time high. Mao Zedong proclaimed the creation of the People’s Republic of China in 1949, and in January 1950, China joined with the Soviet Union to formally recognize the communist Democratic Republic of Vietnam.

During the Cold War, the U.S. practiced a policy of containment. President Harry S. Truman’s Truman Doctrine pledged political, military, and economic assistance to democratic nations facing threats from communist forces. His successor, President Dwight D. Eisenhower, put forth the Domino Theory that a communist victory in Vietnam would create a domino effect in Southeast Asia… and therefore must be prevented at all costs.

“The Vietnam War was at once a war to reconcile issues of European imperialism in a new postcolonial space, a war between Marxism-Leninism and Democratic-Capitalism, and a war between Vietnamese parties,” says Nguyen.

Emperor Bao Dai was succeeded by Catholic nationalist Ngo Dinh Diem. His strong anti-communist stance was popular with the Americans who helped him rise to power. But Diem’s preferential treatment of the Catholic minority led to protests throughout South Vietnam. In May 1963, eight Buddhist protestors were killed by government officials in Hue.

In response, Buddhist monk Thích Quang Duc set himself on fire in the middle of a busy Saigon intersection. Other monks began to immolate themselves in what became known as the “Buddhist crisis.” The United States lost confidence in Diem’s ability to lead.

That November, the United States backed a military coup in which Diem and his brother, Ngo Dinh Nhu, were murdered. (U.S. President John F. Kennedy would be assassinated less than three weeks later.) The coup was followed by a chaotic succession of 12 different governments in South Vietnam between 1963 and 1965.

The Gulf of Tonkin Incident, also known as the U.S.S. Maddox incident, marked the formal entry of the United States into the Vietnam War.

“In the summer of 1964, the Johnson administration was laying secret plans for an expansion of U.S. military involvement in Vietnam. Any such wider action should have congressional support, officials determined, and the Gulf of Tonkin incident provided the opportunity to secure this authorization,” says Fredrik Logevall, Laurence D. Belfer Professor of International Affairs at the John F. Kennedy School of Government at Harvard University.

On August 2, 1964, the U.S.S. Maddox encountered three Soviet-built North Vietnamese torpedo boats in the Gulf of Tonkin. The Maddox fired what it described as warning shots and was met by torpedo and machine gun fire. On August 4, the U.S. destroyer Turner Joy and the U.S.S. Maddox reported that they had been ambushed, though the Turner Joy’s account has since been called into question by historians.