Foreign national courts are categorically prohibited from prosecuting a head of state of another country. From the beginning of the twenty-first century until very recently, this view was nearly unanimous. A 2002 decision of the International Court of Justice, in the Arrest Warrant case, strongly supports it. According to the court, heads of state enjoy ‘full immunity’ from foreign criminal jurisdiction. Thus, the prohibition to prosecute foreign heads of state even extends to those who perpetrate aggression and other war-related crimes. That view is based on a twenty-first century myth. According to the myth, heads of state have long – ‘from time immemorial’ – enjoyed an absolute personal immunity from foreign jurisdiction. This article identifies the origin of the myth and parses through crucial historical facts that disprove it, particularly, the indictments against Hitler as the sitting head of state of Germany, their endorsement by the United Nations War Crimes Commission, and the judgment of the International Military Tribunal. A proper debunking of this myth is not only important as a matter of setting the historical record straight but is also relevant for present-day debates about the prosecution of heads of state (or heads of government) who – like Vladimir Putin, Bashar al-Assad, Min Aung Hlaing, and Benjamin Netanyahu – might be responsible for aggression, genocide, war crimes, crimes against humanity, and other crimes.