What caused the U.S. to send a large number of military advisors to Vietnam?

The United States sent a large number of military advisors to Vietnam primarily due to its policy of containment during the Cold War, which was aimed at preventing the spread of communism. This policy was rooted in the Domino Theory, which held the belief that if one country in a region fell to communism, then the surrounding countries were also likely to fall, like a line of dominos.

In the context of Vietnam, the North was led by Ho Chi Minh and the communist Viet Minh, who sought to unify the country under a communist government after the expulsion of the French colonial powers (who were defeated at the Battle of Dien Bien Phu in 1954). To oppose this, the United States supported the South Vietnamese government, which was non-communist and sought to prevent the spread of communism from the North.

In 1961, President John F. Kennedy increased the number of military advisors to help train and assist the South Vietnamese army (the ARVN). This was partly due to a Cold War strategy of sending advisors and providing military aid to allied nations rather than committing large numbers of American troops. However, as the conflict continued and the situation for the South Vietnamese government became more precarious, the number of advisors continued to grow.

The commitment surged especially after the Gulf of Tonkin incident in 1964, which led to Congress passing the Gulf of Tonkin Resolution, effectively authorizing President Lyndon B. Johnson to use military force in Vietnam without a formal declaration of war. This incident was a catalyst that marked the escalation of U.S. involvement, transitioning from an advisory role to active military engagement, and led to the deployment of hundreds of thousands of U.S. troops to Vietnam by the late 1960s

Understanding the Vietnam War requires understanding the geopolitical climate of the time. The Cold War was a period of tension between the United States and the Soviet Union, along with their respective allies, from the late 1940s through the early 1990s. During this period, the two superpowers sought to spread their political ideologies, with the U.S. promoting capitalism and democracy, and the Soviet Union spreading communism.

The U.S. policy of containment aimed to limit the spread of communism worldwide. This policy led to U.S. involvement in various conflicts, such as the Korean War and the Vietnam War, where they fought to support non-communist governments. American leaders, like President Eisenhower, who first articulated the Domino Theory, believed that the fall of one nation to communism might trigger neighboring countries to fall as well, especially in strategically important regions like Southeast Asia.

Initially, the help provided by the U.S. in Vietnam was part of a broader strategy of giving support to countries that could resist communism from within, supplying them with military equipment, training, and strategic advice. However, as the situation in Vietnam deteriorated, the U.S. found itself more directly involved, leading to a full-scale military conflict that lasted for years and became increasingly unpopular at home, dramatically affecting American society and politics.