The Oxford handbook of the history of international law (2012) contains not a word about papal embargoes. On the showing of this book, it should. Papal prohibitions against trade with perceived enemies of the Roman Church were a formal part of the canon law at least from the Third Lateran Council (1179) to the late eighteenth century. The Council's decree Ita quorundam (X 5.6.6) condemned Christians who carried the tools of war – arms, iron and timber – to the Saracens, threatening offenders with excommunication, confiscation of possessions and even enslavement. The decree was part of papal sponsorship of the Crusades, forbidding a form of trade which hindered Christian victory in the East. This book traces the process by which that decree was taken into the canon law, extended and clarified by later papal decretals, and finally found a home at In coena domini, the papacy's ritual excommunication on Holy Thursday of a long list of malefactors: pirates, falsifiers of papal letters, heretics, appealers from popes to future councils, schismatic Christians, judges who cited clerics to appear before secular courts, and several others. This book might be faulted for lumping them together, treating them as if they were all subject to the embargo imposed on traders with Turks and Sarracens. The medieval canonists did not do this; the offences were defined in different terms and the sanctions were not identical. However, from a functionalist perspective, there is much to be said in favour of the author's approach. Major excommunication cut offenders off from contact with most other Christians. It was not unlike an embargo in its effect. Moreover, this approach fits the theme of this book, one captured by its title. The author sees the ‘rationality’ of these prohibitions not in the harm that they caused infidels and excommunicates. Their true purpose was to preserve the faith of those inside the Church and to cement their confidence in the powers of the papacy. Not for nothing was absolution of violators reserved to the apostolic see. Much of the attention that historians have paid to the papal embargoes has dealt with the question of whether they were effective in practice. Most have concluded that they failed. The author considers this a mistake. Even if one grants that omnibus trade prohibitions and sentences of excommunication did not achieve their stated goals, that does not mean that they failed to achieve a rational purpose – limiting contacts that might ‘endanger the soul’ of faithful Christians. Modern readers may not see much rationality in this, but some understanding of the habits of mind that produced it constitute a benefit of reading this book.