This chapter explores why the USA decided to apply the Geneva Conventions during the Vietnam War. In particular, it examines the US decisions to treat North Vietnamese and Viet Cong detainees as POWs and to investigate war crimes committed by US forces. It begins by providing some historical background regarding US attitudes to perennial defection from IHL obligations and argues that US policy makers and the military were united in their views that the USA was not obliged to apply the Conventions in such cases. It then demonstrates that logic of appropriateness explanations nevertheless do not fully account for the US decision to apply the Conventions. Instead, concerns about positive reciprocity played a crucial role in persuading US decision makers to apply the Geneva Conventions. These concerns were central to US decisions to apply GC III to North Vietnamese and Viet Cong detainees and to investigate alleged US war crimes. In this case, the USA relied on positive reciprocity because it viewed the North Vietnamese and Viet Cong as potential partners in a relation of specific reciprocity.