Reynolds argues that the Baltic Crusades were unique within the crusading movement, not because of geography or because they were waged against pagans rather than Muslims, but because of the integral role that mission and conversion played in them. Moreover, he contends that the Baltic Crusades must be understood not only in the context of the emergence and evolution of crusading ideology over the course of the eleventh and twelfth centuries, but also in the context of a long prehistory in western Christendom of missionary warfare and baptism by treaty.

Chapter 1, “Frankfurt, 1147: The Baltic Crusades Begin,” and chapter 2, “A Peculiar Institution,” set the scene for and narrate the beginning of the Baltic Crusades, which followed a call to arms for the Second Crusade (1147). In chapter 3, “The Crusade Studies Revolution,” and chapter 4, “The Uses of Prehistory,” Reynolds sets out his methodology and places his work in the context of previous scholarship. It is in chapter 4 that he introduces the terms paleo-, meso-, and neo-prehistory and argues for the usefulness of prehistory as a category of inquiry. Having set out his methodology, Reynolds seeks to apply his taxonomy in chapter 5, “‘Missionary War’: Preaching with a Tongue of Iron.” He sees the era from Constantine through St. Augustine as the paleo-prehistory of crusading. The meso-prehistory, he claims, was “ushered in” by the conversion of Barbarian kings like Clovis and reached its apogee with Charlemagne's wars against the Saxons (93). He sees the neo-prehistorical period of the crusades as that of the eleventh-century Reform Papacy and Peace and Truce of God movements (94). While his emphases might be slightly different, Reynolds's perspective here is not a radical departure from what has gone before: even Carl Erdmann, whose eponymous thesis argued that the most important developments on the way to the Crusade idea came in the eleventh century, began The Origin of the Idea of Crusade (trans. Marshall W. Baldwin, [Princeton University Press, 1977]) with a chapter on Constantine's “Holy Banners.”

In chapter 6, “Baptism by Treaty,” Reynolds makes a strong case that the use of baptism by treaty and sword-point conversion are what made the Baltic Crusades a unique phenomenon within the wider crusading movement. In chapter 7, “The Ministry of Swords and Shields,” he pushes this idea further by exploring how punishing apostasy became a defining characteristic of crusading in the Baltic. Chapter 8, “A New Kind of Pilgrim,” helpfully introduces the concept of “White martyrdom,” a kind of career pilgrimage which Reynolds traces to St. Columbanus, founder of the monastery at Iona, and which he sees as more closely related to Baltic Crusading than the kind of temporary pilgrimage which provided the intellectual and spiritual antecedents for Holy Land crusading. In chapter 9, “Christian on Christian Violence,” Reynolds argues that the frequency of intra-Christian violence that emerged during the course of the Baltic Crusades—for instance, violence perpetrated against apostatizing converts—is another feature that set them apart from crusading more generally. The assertion made in chapter 10, “The Legacy of the Baltic Crusades,” that by the mid-fifteenth century crusading had “drifted into obsolescence,” certainly overstates the case, and one is compelled to think of the work of Norman Housley (inter alia) on the later crusades as evidence to the contrary (181). However, Reynolds's discussion in this chapter of the Spanish conquests in the New World is valuable, and his notion that they should be seen as more in the tradition of the Baltic Crusades than the Holy Land Crusades or even the Reconquista, neither of which had a major proselytizing element, is a compelling idea.

The Prehistory of the Crusades is an uneven book. There are many errors in citation and Reynolds quotes far too often from secondary material. More troubling is the fact that the book often seems an exercise in apologetics for the Crusades. Quite typical is the following expression of a desire to recuperate them as a positive good: “But to decry their [the Baltic crusaders’] violent practices in what was generally a very violent era is to simultaneously ignore their eventual positive benefits” (25–26). At times this orientation borders on chauvinism, such as when Reynolds characterizes the violence of crusaders against pagans as “an act of mimicry in which the putatively higher Christian standard was accommodated to the lower one” (126). Elsewhere, he writes of the “corrosive influence of native customs on the Christians” (140).

Nevertheless, the book has undeniable merits. Intended for a general audience, it is written in a brisk, lively voice, and Reynolds excels at narrating events, such as when setting the scene at the beginning of a chapter, for instance. Moreover, his claims about the unique missionary war quality of crusading in the Baltic are convincing and thought-provoking, a valuable reminder that the Baltic Crusades were sui generis not only in their geography and their target, but also in their purpose. Finally, Reynolds is entirely correct in his assessment that the Baltic Crusades remain understudied: English-language works, and those intended for a general readership, are lamentably few and far between. Any work that seeks to remedy the situation, as this one does, comes as a welcome contribution to the field.