“What is going on, is destroying not only a nation; it destroys, I believe, your life-work; it destroys everything which I have believed you stand for …” With these words, written in the last days of 1966, the American scholar John Fried implored his famous uncle Hans Kelsen to take a stand in the emerging debate about whether the American escalation in the Vietnam War was against international law. In his lecture, Samuel Moyn (Yale University) sheds light on the historical and legal context of this remarkable letter, which also offers some insights of contemporary relevance. The keynote lecture was held at the opening symposium of the Cologne Center for Advanced Studies in International History and Law (CHL) on 16 September 2024.