There has seldom been a better time to be a scholar of the Vikings. New discoveries and processes in archaeology have opened up rich fields of debate about the lives and activities of the inhabitants of what is typically referred to as the "Viking World." New theoretical approaches, such as diaspora theory, migration theory, and network theory are helping us to conceptualize this period in novel ways, moving away from older debates about violence and allowing us to instead investigate the myriad ways in which the Vikings (or more accurately, the Norse) engaged with the wider world.  And yet for all this attention and vibrant scholarship, there remains a surprising gap in the historiography of the Viking Age. Numerous books cover the Viking Age in the North Atlantic, in England and Ireland, and in Scandinavia. When it comes to the Continent, however, there has never been a single focused English monograph on the topic. Scholars have had to work through a wide body of articles and edited collections, doing most of the heavy lifting for themselves, in order to get a clear sense of the Viking impact on the Carolingian Empire. It is this gap that Christian Cooijmans' book, Monarchs and Hydrarchs, seeks to address by providing a coherent synthesis and analysis of Viking activity across Francia from the late 8th to the mid-10th century.
In some ways, Cooijmans' book is a nod to that classic debate over Viking violence that has occupied the field since at least the 1960s. Indeed, it is with this debate that he begins the introduction to his work in a section titled "The story so far." But for Cooijmans the question is not whether Vikings engaged in violence (they did) or whether that violence was "normal" or "abnormal." Rather, the point of this work is to understand the methods and approaches of Viking violence, or, to put it more broadly, as Cooijmans does, "viking activity."  This may seem like a small distinction, but by shifting from violence or raiding to "activity" more broadly conceived he is able to encompass a much wider array of behavior, including not just raids on monasteries but also the logistics of long-term encampments in the late 9th century, and even the all-important system of commendation that frequently marked Franco-Scandinavian interaction. But rather than merely building a catalog or narrative of Viking activity in the Frankish world, what this work is undertaking, to put it in the author's own words, is the construction of a "conceptual development model" which will allow for the creation of "an abstract, phased paradigm of archetypal activity based on the compiled and collated data from the primary sources" (4). At its heart, what Cooijmans is seeking to do is to lay out the core patterns and structures that dictated where, when, and (most importantly) how and why Norse peoples engaged in Viking activity across Francia.
So, how does Cooijmans go about constructing this model? After setting up some of the historiographical background in his first chapter, chapter 2 begins by focusing on "The Scandinavian perspective." This chapter lays out some of the key transformations, both political and economic, that set the stage for the Viking Age. The use of both textual and archaeological data effectively highlights the key importance of this sort of cross-disciplinary work in the field. Two key developments are highlighted here: the growth of political centralization on the one hand, and the burgeoning trade networks of the North Sea and Baltic on the other. In whole this chapter serves as a good introduction to the state of Scandinavia in this period and provides a sense of the drivers that led to Viking activity both in the Frankish world and more broadly.
Of more importance for the work, and for the field more broadly, is the time spent in this chapter introducing the titular hydrarchs who will feature so prominently in this work going forward. The term itself originates from work on the Early Modern Atlantic by the historians Peter Linebaugh and Marcus Rediker and refers to, in Cooijmans' words, collections of Viking fleets which were, "unaffiliated, self-governing networks of political power – small-scale shipborne societies beyond the grasp of sovereign and commercial authorities" (32). While Cooijmans is not the first scholar of the Viking Age to make use of the concept this work is the first full-scale monograph to center the hydrarchy as fundamental to understanding the Viking Age. It is the flexibility of the hydrarchy system, centered on fleets able to move quickly, to grow or shrink as needed, to share and disseminate information broadly, and to adapt to changing circumstances over time that is key to unraveling the patterns of Viking activity on the Continent.
Having established the Scandinavian context and set up his key structure for understanding the organization of Vikings abroad Cooijmans turns his attention to the Carolingian empire, where the remainder of the book will have its focus. Chapter 3 provides a brief overview of the Carolingian world, effectively a parallel to the preceding chapter on Scandinavia, again focusing on political and economic structures. A quick overview of the political context (notably the division of the empire in 843) is provided and some of the key economic and administrative structures which would be impacted by the arrival of the Vikings are laid out. At its heart, this chapter can be understood to primarily be about setting up the playing field for those Scandinavians who we met in Chapter 2.
Chapters 4 and 5 approach the main focus of the work as Cooijmans lays out Franco-Scandinavian interactions up to the mid-9th century. These chapters are quite short (12 and 20 pages respectively, including notes) and serve more to set the stage for later Viking activity than as a focus in their own right. Chapter 4 provides a brief discussion of early reports of raiding along coastal sites and some discussion of how trade and key diplomatic interactions might have served as vectors for information transfer necessary for early Viking hydrarchies to get their start on the Continent. Chapter 5 give us a clear sense of the uptick in intensity witnessed during the last years of Charlemagne's reign and the first decades of Louis the Pious' with some nice discussion of the evolution of practices. In unpacking the potential significance of this early activity Cooijmans demonstrates his ability to take often sparse or stubborn primary sources and wrest potential patterns and logic from them.
In chapter 6 of the book, "Intensified Viking Activity" Cooijmans really hits his stride. The chapter stands at almost one hundred pages, nearly a third of the length of the book, and it is here that we can really see Cooijmans' focus and his goal of accumulating as much data as possible in order to successfully construct a model that can be applied to Viking activity not just on a case by case or location by location basis, but across the entirety of the Frankish world. Detailed sections on the raids themselves, on the practice of encampment, and on the various means by which the Carolingians sought to prevent or blunt Norse predation encompass a wide array of sources and nicely cover the range of Viking activities during the height of the Viking Age. Central to all of this activity is the idea of hydrarchy, that flexible system which allowed numerous Viking bands to operate across key regions of the Frankish realm, to engage not just in raiding but in tribute collection, in mercenary work, in trade, and even in the very political system of the Franks themselves. Working through this chapter can be somewhat daunting given its size and depth and the existence of numerous subsections (and even sub-subsections) can sometimes make it hard to establish a clear narrative of the period, but the result is a comprehensive overview of the Vikings not just as violent invaders but as rational actors with an increasing familiarity with the Frankish world and the willingness to adapt as circumstances warranted.
Having accumulated and run through his data Cooijmans' final major chapter shifts back from the material itself to the larger conceptual model at the heart of his investigation. In this chapter he puts forth the second thrust of his argument (beyond the importance of hydrarchy), dividing Franco-Scandinavian interaction into a series of four phases and arguing for an "archetypal sequence of Scandinavian encroachment across the Frankish realm" (209). In Phase One, politico-economic pressure at home and from abroad creates connections between Scandinavia and the Continent and spurs the initial formation of hydrarchies. In Phase Two, Viking activity impinges on the periphery of the Frankish world and we see some limited political links forming between Franks and Norsemen. Spurred on by early successes, Phase Three sees a major expansion of Viking activity beyond coasts, further inland and marks the rise of encampments and even full-scale Scandinavian entry into the Frankish world via the granting of various benefices. Finally, in Phase Four the hydrarchies peter out, the victims of their own success and of a changing landscape both at home and abroad in the Frankish world. Having established this sequence, the rest of the chapter works to demonstrate the validity of this sequence in the three key theaters of Viking activity, the Lower Rhine, Seine, and Loire basins. The choice to split these regions apart and to demand that each phase be visible in each area feels a bit artificial given his earlier point about the flexibility and connectivity of hydrarchies of Viking networks across wide areas, but all in all the model works well and makes sense as a way of conceptualizing how Viking activity unfolded across the period. Cooijmans closes the chapter with the guarded but optimistic statement that "the conceptual development model may be provisionally approved, making it the first functional schema of its kind to deal with the reach and repercussions of viking activity. Whilst acknowledging that individual Scandinavian hosts acted with forethought and intent, this innovative model demonstrates hydrarchy as a whole to have adhered to an overarching paradigm of regional movement and encroachment" (228).
All in all, the work that Cooijmans has done in this book is meticulous and well thought out. Having constructed and validated his model to his satisfaction, the book then closes with some very brief remarks (4 pages in total) about the value of the model and its potential utility for investigating other areas of the Frankish world such as the Garonne Valley, and pushes for the use of the hydrarchy as a tool for thinking about Norse activity across the Viking World more broadly. Of the two, I can't help but feel that the hydrarchy approach will be the more popular and useful contribution to the field as a whole given its applicability not just to the Frankish concept but more broadly across the Viking World. But for scholars of Francia the conceptual model has the potential to expand both the analysis of extant data in new ways and to help fill in of gaps in the historical record that plague the study of Viking activity on the Continent. Cooijmans is not shy about the importance of deductive reasoning when working with our often spotty or heavily biased medieval sources, and in this practice the model will be of tremendous use.
There is perhaps only one major critique worth highlighting at the close of this review. In my introduction I noted that Cooijmans is working to fill a key gap in the historiography of the Viking Age by bringing Francia to the forefront. This is true, but it is worth couching this to a certain extent because what Cooijmans cares about, first and foremost, is still Viking activity. To be fair, he is never shy about this focus, but in his push to create a conceptual model for this Viking activity in particular it does feel at times that certain key engagements between Franks and Norsemen are left by the way side. For any substantive discussion of the diplomatic conferences of the late 8th and early 9th century, for instance, one must turn to Pierre Bauduin's Le Monde franc et les Vikings.  It would have been nice to see a bit more discussion of the titular monarchs of Scandinavia as well as the hydrarchs and to see how the interplay of elements such as Anskar's missions, the on-going relationship between the Carolingians and the kings of Denmark, or the shifting internal politics of the later Frankish kings fit into and impacted the model that he is constructing. What we get is a very clear and detailed picture of Viking activity in Francia but not always as clear a picture of the larger context or significance of that activity more broadly.
Regardless, Monarchs and Hydrarchs will undoubtably push the study and discussion of Franco-Scandinavian interaction forward. Beyond the thorough analysis itself, the book is replete with useful charts, tables, and maps which will delight both graduate students and established scholars alike. No more will each individual have to scour the numerous annals to compile a list of attacks, encampments, and tribute payments, for instance. In this, Cooijmans has given a gift to the field. Above all, no work of this detail and breadth exists and it was sorely needed. With his thoughtful analysis of the available data, and his ability to read between the lines and extrapolate out patterns, Cooijmans exposes the complexity of Viking activity on the Continent. He champions new ways of conceptualizing Viking activity and organization, centered on the agency, rationality, and flexibility of the Norse. The work is without a doubt a must read whether one is specifically interested in Franco-Scandinavian engagement or in the Viking Age as a whole.
1. Lesley Abrams, "Diaspora and Identity in the Viking Age," Early Medieval Europe 20, no. 1 (February 1, 2012): 17-38; Judith Jesch, The Viking Diaspora (London: Routledge, 2015); Shane McLeod, The Beginning of Scandinavian Settlement in England: The Viking "Great Army" and Early Settlers, c. 865-900 (Turnhout: Brepols, 2014); Søren Sindbaek, "Crafting Networks in Viking Towns," Medieval and Modern Matters 4 (2013): 119-132.
2. Cooijmans choice in his work to leave "viking" uncapitalized points to a subtle but largely unaddressed debate in the field over how (and whether) to disentangle raiding from identity. This review article will default to the standard practice of capitalizing except when quoting Cooijmans directly.
3. Pierre Bauduin, Le monde franc et les Vikings: VIIIe-Xe siècle (Paris: Albin Michel, 2009).