The main reason why the United States of America failed to defeat Vietnam between 1963 and 1975 was largely due to the superior military tactics of the Viet Cong - NSC History - Question 4 - 2017 - Paper 1

The United States entered the Vietnam War primarily to prevent the spread of communism as part of the domino theory. The US sought to support the South Vietnamese government against the communist Viet Cong.

The Viet Cong employed guerrilla warfare tactics which included ambushes, booby traps, and hit-and-run attacks that were particularly effective against the conventional forces of the US army. Their knowledge of the terrain also provided a strategic advantage.

The Gulf of Tonkin Incident in 1964 led to escalated US involvement, showcasing how the Vietnamese response to American aggression was to spread military operations and bolster their own efforts. This incident prompted the US Congress to grant President Johnson overwhelming support to escalate the war.

In conclusion, while the military tactics of the Viet Cong were a crucial factor in the US failure in Vietnam, they were intertwined with political, military, and public sentiments. A comprehensive understanding of this complex conflict highlights that it was not solely the Viet Cong's tactics but also the US's strategic miscalculations and lack of understanding of the Vietnamese context that led to the eventual outcome.