Despite the vast historiographical tradition concerning the history of the Hundred Years’ War, little attention has been paid overall to the role of the papacy as a political actor in the Anglo-French conflict, while mentions of the papal diplomatic mediation between England and France are limited to individual popes, who intervened in the conflict at different stages. Accordingly, historians working on the Avignon papacy have addressed only briefly papal involvement and arbitration in the Anglo-French conflict, while an exhaustive assessment of papal involvement in the Hundred Years’ War covering the period 1337–1453 still remains to be published.

Willershausen's monograph tries to remedy this neglect, focusing on papal intervention in the Anglo-French conflict during the Avingon period (1337–78). As the title of the book points out, Willershausen is especially interested in the patterns of papal diplomatic activity and practice, which, from the outbreak of the conflict in 1337, aimed at pacifying the warring parties by virtue of thirteenth-century papal claims to plenitude of power (viae pacis). Willershausen's work is organised in three main sections: the sources, both documentary and narrative, which evidence the papal peace-making efforts; papal interventions in the Anglo-French conflict over the period 1337–78, which are addressed chronologically; and the modalities of papal diplomacy with a specific concern for the personal role of the pope and his representatives, the channels of political and symbolic communication between the Apostolic See and the addressees, and the forms employed in peace-making (notably, truces, peace agreements and peace conferences).

Willershausen pays special attention to the examination of contemporary chronicles, which are carefully compared and deconstructed in order to assess papal involvement in the Anglo-French conflict. This is particularly the case for the chapter dedicated to the peace-making efforts of Cardinals Tayllerand de Périgod and Niccolò Capocci at the time of the battle of Poitiers in September 1360, where Willershausen reassesses the negative rhetoric concerning the cardinals’ mediation expressed in the fourteenth-century chronicles. Conversely, while extensively employing the papal registers, Willershausen does not make as much use of English and French diplomatic correspondence, especially well-preserved for this period in the collections at The National Archives in London and the Archives Nationales in Paris. Willershausen is in fact more interested in adopting sociological communication theories to assess the discourse between the papacy and its counterparts than in looking at the actual diplomatic communication among the parties, which is evidenced in the diplomatic correspondence between the Apostolic See, its nuncios and political addressees and still preserved in English and French archives. However, as historians of English medieval diplomacy such as Cuttino and Pierre Chaplais have pointed out from the 1940s onwards, the latter provide a complete and well-rounded picture of the discourse between the papacy, England and France at the time of the Hundred Years’ War alongside the narrative sources.

Meanwhile, in his chronological examination of the papal mediation of the Anglo-French conflict, Willershausen tries to find patterns of behaviour and looks at the development of a papal strategy throughout the period with regard to: the attempts of Benedict xii to avoid and stop the conflict; the lengthy organisation of the unsuccessful peace conference in Avignon in 1344, when, in Willershausen's opinion, despite their efforts the papacy and his representatives achieved little control over the negotiations; the papal shuttle diplomacy after the English victory at Crécy in 1346, when Clement vi was slow to react to the high-quality information provided by his nuncios; the failed papal attempts at establishing an Anglo-French peace, overseen at Guines in 1354 by Guy of Boulogne, whose efforts are positively assessed, challenging established historiographical interpretations; the intervention of the papal nuncios before and after the battle of Poitiers in 1356, when papal peace-making efforts ultimately led to the Treaty of Brétigny-Calais in 1360; and, finally, papal involvement in the peace conference at Bruges in 1375–7. Two main agencies are here at the core of Willershausen's analysis: the role of individual popes, who intervened as mediators in the Anglo-French conflict, initially as private persons and, from the time of the peace of Brétigny-Calais, as guarantors of peace among the parties; and the activity of papal envoys and representatives. As Willershausen noted, throughout the period under examination the latter were interestingly appointed as nuncios rather than legates a latere in order to suit better the private nature of the papal intervention in the Anglo-French conflict. Furthermore, the papal representatives were chosen not only among the cardinals but also among the clergy of lower rank because of their specific expertise and were given various faculties to mediate between England and France in accordance with the specific needs of each individual diplomatic mission. In this respect, Willershausen tries to underplay the importance of the nuncios’ background in the papal choice of its diplomatic envoys, maintaining, against recent historiographical interpretations, the private nature of the fourteenth-century popes’ interventions in the Anglo-French conflict. As Willershausen argues in the last section of his book, we should in fact distinguish the pope's and his representatives’ private mediation in the Hundred Years’ War from the public role of the Apostolic See as an institution, which was seen by the time of the peace of Brétigny-Calais as guarantor of peace among the parties and which was reaffirmed in the contemporary papal interventions concerning the clash between the Church and Crown in England over provision to ecclesiastical benefices. However, although suggestive and worth noting, this interpretation seems difficult to prove through the surviving evidence, as fourteenth-century papal documents and other sources do not really draw a clear and definitive distinction between the pope and his representatives as individuals and the Apostolic See as an office.

All in all, despite some of its controversial claims Willershausen's book has to be praised as the only comprehensive recent study which specifically addresses papal involvement in the first phase of the Hundred Years’ War. Whether or not one agrees with the author's conclusions, the book evidently challenges the existing historiography and revises established historiographical interpretations, especially through the careful and fresh reading of fourteenth-century chronicles. On a presentation level, Willershausen's style still echoes the format of a doctoral dissertation and there is some confusion on details in the footnotes and the index, where names of persons and places are occasionally not included. Despite this criticism, the book has to be listed among the standard works which should be read and cited on this topic.