The authors of this systematically argued book have long experience studying terrorism and other forms of political violence. In Underground Violence: On the Nature of Terrorism, Luis De la Calle and Ignacio Sánchez-Cuenca reflect on one of the four main areas of research on terrorism (i.e. its definition), but in ways that have potential implications for understanding the other three (causation, consequences, and response).

The book has a consistent argument running through it, namely: that what renders terrorism unique “is the underground nature of its violence”, which “does not require any control of the territory in which the deed takes place” (p. vi). Luis De la Calle and Ignacio Sánchez-Cuenca write that ‘the defining feature of terrorism is the fact that this violence is politically motivated and perpetrated underground, under conditions of clandestinity’ (p. 4). This is not, in itself, a novel argument, and the authors are open about that (p. 4). They aim to develop and test this thesis, and they do that through a combination of quantitative analysis and exploratory case studies (the latter focusing on the Tupamaros in Uruguay, the Basque separatist group ETA, the Shining Path in Peru, Palestinian terrorism, and ISIS in Iraq and Syria).

The book aims to be more than descriptive but rather has an explanatory goal, that is, to explain why some groups at times carry out attacks of one kind (clandestine, terroristic) rather than of another (more open, insurgent). In the authors’ words, “The critical question is under what conditions rebel leaders choose terrorism over other armed strategies”. Luis De la Calle and Ignacio Sánchez-Cuenca argue that ‘clandestine violence occurs when there is a large asymmetry between the actors involved in the violence. In asymmetric settings, the weaker side (usually, the rebels) will act covertly because of their lack of territorial control’ (p. 8). The book acknowledges that territory-controlling organizations involved in insurgency also carry out some terrorism, claiming that these groups pursue the latter in areas where they do not possess territory.

There is a lengthy, well-known tradition of scholars studying terrorism definition, with figures as diverse as Alex Schmid, Stathis Kalyvas, Ben Saul, and Anthony Richards significantly involved in this endeavor. Of course, many definitions different from that offered here by De la Calle and Sánchez-Cuenca would still allow for recognition that territorial control and the asymmetries of power have some effect on groups’ choice of violence.

Moreover, some readers will consider that territorial control might not be as decisive as these authors suggest in relation to decisions about modes of political violence. De la Calle and Sánchez-Cuenca argue that ‘The production of violence is ultimately determined by the territorial control (or lack of it) of the armed group’ (p. 13). But other determinants matter too. The late Hamas leader Ismail Haniyeh, for example, said that the group’s choice about the scale of attacks on Israel was determined by levels of popular support for such violence. So what the authors here call ‘the generative mechanism of terrorism’ (p. 14) in fact involves more than just control over territory, and is often a multi-causal, complex process leading to terrorist groups’ decisions about the type of violence that they use, and when and where.

Another response to this thoughtfully argued book is that some dynamics of state action might have been more extensively considered within it. The authors suggest that “Terrorism is intrinsically linked to secret, covert action” (p. 38). But, so too is spying. Yet, state security agencies are part of an organization (the state) that controls territory; they also on occasion practice violence which has an unquestionably political motive. De la Calle and Sánchez-Cuenca suggest that “Terrorism is politically motivated violence carried out secretly by actors without territorial control” (p. 38). But, if what makes such violence terrorism is that it is clandestine and underground (the central claim of this book), then what of clandestine, underground violence for political cause practiced by state actors, much of it within the territory that the state controls? The authors briefly seem to acknowledge this point, but it is not discussed at length in their book. So they admit that not all terrorist violence is carried out by underground groups (“groups with territorial control can also employ terrorist tactics” (p. 64)), but their discussion focuses more on guerrilla groups than on state actors. Yet many cases exist where groups that are part of the state act secretly, immediately, clandestinely, and violently within that state’s territory. Here, the explanation for such violence cannot be a lack of territory (as would be suggested by the central thesis of this book), yet the violence is clearly both clandestine and political. It is also carried out by an actor who tends to be the stronger in the relevant conflict. This conflicts with the authors’ claim that ‘Terrorism is chosen by the weaker party when the conditions under which it acts are maximally asymmetric’ (p. 77).

Ultimately, it might be argued that the roots of terrorist violence are more tangled than decisively explained by territorial control and that it is not quite true with terrorist violence that ‘the choice of tactics is a function of the degree of territorial control’ (p. 101). Again, the authors occasionally seem to accept this qualifying challenge to their central thesis, alluding to the greater complexity of the explanation behind Fatah’s tactic selection, for example (pp. 160-1).

There remains much to admire about the book, and about the methodological approach adopted within it. The authors are rightly skeptical about existing terrorism databases, though also sensibly prepared to use them with care: ‘Many quantitative studies on terrorism do not problematize the use of existing datasets’ (p. 113); ‘We are skeptical about findings that have been obtained by uncritical use of the existing datasets on terrorism’ (p. 191). They also offer an interesting (again, usefully skeptical) reflection on definitions of terrorism that focus on the targeting of civilians, and also about those which define terrorism as coercive violence.

In the end, I am not quite persuaded by their central thesis, nor by the authors’ claim regarding definition and terrorism that, ‘The lack of a shared conceptualization has prevented a cumulative body of knowledge’ (p. 5). Despite definitional divergence, there is enough shared ground between many scholars who study terrorism to have allowed for considerable progress to be achieved in our understanding of definition, causation, consequences, and response. Underground Violence: On the Nature of Terrorism will illuminate readers’ understanding further, and it contributes rigorously to ongoing debates on this important subject.