Different people write about the history of international law differently and for different reasons, inspired by the wish to put their international historical legal scholarship at the service of different causes. Some approach this study with a sense of moral duty to those whom the promise of international law has left betrayed or ignored and who are hopelessly wondering what international law’s wrong turn or inaction has practically to do with their current dismal fate. On other occasions, international legal history is approached as part of an effort at international legal norm-entrepreneurship with a view to reforming a specific element in the international legal order with a statistically proven ability to save lives or reduce the likelihood of harm if they are faithfully complied with. In other cases, international legal history serves to trace parallelisms between past scenarios and present-day concerns in order to tailor, through trial and error, better policy choices than those delivered by the earlier adoption of a particular international legal course rather than other alternatives. At other times, thinking backwards and forwards in time may help one to see with greater acuity what particular style of scholarly treatment a subject deserves in order to render justice to the victims of past abuses, for impending crises to be timely deactivated, or for ongoing global threats to be handled in the most constructive and orderly possible manner. Sometimes, retrieving the historical memory of international law contributes to raising consciousness of the root causes underlying certain recurrent phenomena of international concern while catering for the intellectual and ethical betterment – although these two do not necessarily follow each other – of its contemporary protagonists in the light of less than edifying examples of some of their predecessors. While referring to different themes of historical investigation and making use of a diversity of historical methods, many of these and many other uses of historical research in international law perform a critical function. This is because by delving into the rich reservoir of accumulated – but never unequivocal – knowledge of international law that the past has bequeathed to us they intend to elicit, to different degrees and on different scales, a change or transformation in the present status quo inspired by an esprit critique d’internationalité.