Power and Authority, A Trial of Two Swords: A History of the Union of the Holy Roman Empire and the Kingdom of Sicily (1186–1250), by Willem J. Zwalve

The marriage of Henry of Staufen, the eldest son of Emperor Frederick Barbarossa and already king-designate of Germany, and Constance of Sicily in 1186 was to lead to the union of the western empire and the kingdom of Sicily under one ruler. That union proved to be increasingly unacceptable to successive popes, who feared the encirclement and political emasculation of the papacy, and ultimately to an increasingly bitter conflict between the papacy and Emperor Frederick II—the son of the 1186 marriage. While this struggle played a central role in the politics of thirteenth-century Europe, and most aspects of it have been extensively discussed by scholars, an overarching study—and not just confined to Frederick II—is still desirable, not least because some of what is still the key specialist literature was written long ago. Unfortunately, Willem Zwalve does not quite provide this, primarily because he seems to be aiming this book at two different audiences. On the one hand there is a long introduction, of over a hundred pages, which provides a narrative of the history of the Western Empire from the coronation of Charlemagne onwards, as well as a lengthy ‘aftermath’ devoted to events after 1250, especially the reign of Charles of Anjou as king of Sicily and the Vespers revolt, and an epilogue discussing the pontificate of Boniface VIII. These sections, and especially the introduction, appear to be aimed at the general reader with little or no prior knowledge, and are neither original nor penetrating. Nor indeed are they very relevant to the book’s ostensible subject. The core of this work, however, is a scholarly discussion of the deposition of Frederick II pronounced by Innocent IV at the Council of Lyons in 1245, which looks not just at the complex diplomacy involved but at the legal precedents and arguments put forward by both sides in the dispute. This part of the book is more useful, and even those thoroughly conversant with the topic will profit from it.

One should also point to some peculiarities of Zwalve’s method. His approach, apart from in the discussion of the Council of Lyons, is largely chronological. He has read a very wide range of contemporary sources, although most of these are narrative, which are quoted extensively in the footnotes—albeit not always from modern editions. But his use of these sources is often uncritical, and this leads to a number of misapprehensions and outright errors. His use of documentary sources also leaves something to be desired. He appears, for example, to be unaware of the new edition of the documents of Frederick II, of which six volumes have now been published and which stretches as far as 1231. He relies exclusively on the nineteenth-century editions of Huillard-Bréholles, Winkelmann and Weiland, although to be fair his use of these is thorough. The discussion of papal–imperial diplomacy also makes extensive use of Rodenberg’s edition of documents from the papal registers, and he displays great erudition in discussing canon law sources, but he also seems unaware of the modern edition of Innocent III’s register (the first volume of which did after all appear as long ago as 1964). Moreover, he confines his footnotes solely to the citation of primary sources, and it is therefore hard to know how far he has engaged with the scholarly literature. The bibliography appended, which cites many quite dated studies, and omits some very relevant modern ones, about the papacy, Crusades against Christians and the kingdom of Sicily, unfortunately suggests not very much.

One should note that the author is not a professional historian but a distinguished professor of law and legal history, and the strengths of this book reflect its author’s background. The discussion of the Council of Lyons and of the canonist underpinnings of the condemnation of Frederick II there is valuable, and students of the diplomacy of the struggle will find the extensive citation of source material in the footnotes useful, even if the author’s view of the thirteenth-century popes tends to be one-dimensional, and suggests greater consistency in their policy than was in fact the case. But for a more nuanced and original view of the papal–imperial conflict, one would instead recommend Brett Edward Whalen, The Two Powers: The Papacy, the Empire and the Struggle for Sovereignty in the Thirteenth Century (2019), which has a narrower chronological focus, but a much more interesting and sophisticated approach.