Final answer:
The McNamara fallacy, or quantitative fallacy, contributed to the US defeat in the Vietnam War by creating a misleading perception of progress based on statistics while disregarding the realities on the ground.
Step-by-step explanation:
The McNamara fallacy, also known as the quantitative fallacy, refers to the reliance on quantitative data and metrics without considering the larger context. In the context of the Vietnam War, the McNamara fallacy contributed to the US defeat by creating a misleading perception of progress based on statistics and metrics, while disregarding the realities on the ground. Despite statistical indicators of progress, the US military failed to consider the impact of guerrilla warfare, the commitment of the Vietcong, and the lack of support from the local population, which ultimately led to the defeat.