Crusade historians have long been divided between those traditionalists who classify the crusades as necessarily focused upon the recovery and defence of the Holy Land between 1095 and 1291, and pluralists who include a wider chronology and broader theatres such as the Iberian Peninsula, Provence and the Baltic. Focusing on the latter, Burnam W.Reynolds argues that the term ‘Baltic crusades’ is ultimately misleading. Often neglected as a sideshow to the main event, they should rather be interpreted through a prehistory extending as far back as the reign of Constantine the Great in the fourth century. The prehistory of the crusades is not an attempt to replace earlier histories of the Baltic crusades – such as those by William Urban and Eric Christiansen. What Reynolds provides, rather, is an interpretation of them through their historical contextualisation and linkage with past precedents. It is upon this basis that he is able to propose his categorisation of them as missionskrieg, or missionary war, a term first coined by Carl Erdmann in 1935.

Reynolds divides his book into ten chapters under three sections: ‘The Problem’; ‘The Method’; and ‘The Application’, followed by a discussion of their legacy. The first part provides an overview of the Baltic crusades, particularly of their origins as a movement under the influence of Bernard of Clairvaux in 1147, who declared that ‘the war against the pagans should continue until such a time as, by God's help, they shall be either converted or wiped out’ (p.21). While initial efforts were as fruitless as the Second Crusade itself, the leadership of Bishop Albert von Buxhövden in the early thirteenth century was much more effective, particularly with the establishment of the twice-annual reisens as a regular feature of these ‘eternal Crusades’. In contrast to the usual interpretations of the Baltic crusades as either an extension of the crux transmarina or a cynical land grab, Reynolds offers the descriptive of missionskrieg to the Baltic Crusades in light of their central objectives of defending new Christian communities while punishing the pagan Balts for their apostasy and their martyrdom of Christians.

In ‘The Method’, the reader is introduced to the debate amongst crusades historians about what qualifies as a crusade. For Reynolds, the immediate question is: ‘Can a campaign against pagans rather than Muslims in a territory far removed from the holy sites of Palestine qualify as a crusade?’ (p.45) Another point of debate is whether the crusading era's inception began with Pope Urban ii’s appeal at Clermont in 1095, or was this a trigger to an already existing development? Was crusading something new, or was it ‘an enlarged continuation of long-existing struggles and institutions’? (p.49) Referencing the work of Paul Cheveddon and others, Reynolds notes the campaigns in Sicily and Spain prior to the Crusades and argues that this prehistory can be traced even to the reign of Constantine, the first Christian Roman emperor.

The meat of the book is to be found in the third part, ‘The Application’. It is here that Reynolds applies the concept of prehistory to the Baltic crusades. Themes include missionary war itself, baptism by treaty, the ‘ministry of shields and swords’ (quoting Henry of Livonia), the application of the term peregrini in the Baltic Crusade context and the necessity of geographical sacralisation, and the further problem of conflict between Christian Germans, Danes and Russians. As an example of Reynold's use of prehistory, he locates the origins of baptism by treaty to patristic theological justifications for infant baptism. Arguments by Augustine evolved into an acceptance that whole peoples could be baptised into Christianity first and catechised later. Non-Byzantine Christian rulers including Charlemagne, Alfred and St Olaf all utilised this approach. If the end justifies the means, a change in religion was fundamental to a change in pagan identity and thus their ultimate inclusion within Christendom.

The appropriateness of the term missionskrieg, for Reynolds, turns on the question of whether the participants understood war itself as a means of converting pagans, or if it really was fundamentally interpreted as defensive – protecting newly planted churches from violently hostile pagan neighbours. We must, argues Reynolds, distinguish between process and end product and conclude that the Baltic crusades were founded upon defensive principles to protect Christian peoples, albeit this process is too often eclipsed by the violent practices inherent to warfare. While Reynolds traces the formation of Christian just war theory from Augustine to the Crusades, he also considers the syncretistic dangers whereby Crusaders adopted ‘atrocious local practices’.

Reynolds ends his study with a chapter devoted to the legacy of the Baltic crusades and missionary war following the Battle of Tannenberg in 1410 and the Council of Constance (1414–18). For him, their legacy lies less in the realm of practice (such as the Spanish conquest of the New World) and more in the realm of application theory. He sees, in fact, a moral lesson for contemporary civilisation. This includes the danger of adopting local violence, but the larger threat is the ‘slippery slope from defense of the innocents to outright conquest in offensive assault’ (p.193). Reynolds concludes that the ‘legacy of Baltic missionary war is therefore a cautionary one. The study of its causes, implementation, and prehistory may supply the perspective needed to avoid the same pitfalls experienced by those on the reisens’ (p.192).

The prehistory of the crusades is a well-researched and innovative study of the Baltic crusades and the origins of unique features that characterised them. Reynolds is convincing in his argument that the crusades did not come into existence in a void, nor that the Baltic crusades were without historical precedence in many of their central traits. His study helps to expand our appreciation of the Baltic crusades from a mere appendage of crusading history by demonstrating both their unique context as well as their prehistory in early medieval society. Helpful in this regard are the study questions provided at the end of each chapter, as well as the glossary of people and terms at the end of the book. In discussing the legacy of the Baltic crusades, Reynolds is thought-provoking and wise in warning Western leaders of the need to learn from the lessons of history lest we repeat them. Overall, this is an interesting and valuable contribution to crusading historiography and will be sure to provoke further discussion.