In the early 1950s nuclear deterrence developed as a guiding principle, in both theory and practice, of military strategy for great powers divided by an emergent Cold War. In Britain influential strategists such as Sir John Slessor campaigned in favour of a “Great Deterrent” in the form of a nuclear counter-threat against the Soviet Union. In 1954 the U.S. Secretary of State John Foster Dulles spoke of the need to deter aggression through a policy of “massive retaliation”—a promise of an overwhelming military response to any form of aggression (as quoted in Lawrence Freedman, “The First Two Generations of Nuclear Strategists,” 1986. In Peter Paret (ed.) Makers of Modern Strategy: From Machiavelli to the Nuclear Age: 740).

This principle of limiting threat through deterrence continued to dominate the security discourse of the Cold War and was seen as the core paradigm of the nuclear age. With the demise of the Soviet Union and emergence of transnational (and mostly religiously inspired) terrorism as the West’s primary security concern, notions of deterrence, however, became widely accepted as irrelevant.

Alex Wilner dissects this “deterrence scepticism” (p. 2) in the first chapter of his book Deterring Rational Fanatics. For the current wave of transnational terrorism, he argues, this dismissal of deterrence can be summarised as follows: Religious terrorists do not fear death and are not conventionally rational; their goals are not achievable or negotiable; they offer few tangible “hard targets” on which to focus retaliation; and have not traditionally avoided vastly superior targets, such as the United States.

His core argument, in turn, is that the principles and logic of deterrence theory (DT)—adapted for a new security environment—continue, upon closer scrutiny, to be valid for policy makers, strategists, and scholars alike. Terrorists, Wilner argues, are “rational fanatics”—calculating combatants, not crazed killers (p. 7). Deterring terrorism, therefore, is feasible if coercion and deterrence are focused on specific roles, components, and values that underpin non-state terrorism.

In Chapter 2, Wilner seeks to adapt DT to these new realities. Deterrence, he argues, consists of convincing or manipulating an adversary to refrain from action as costs are seen to outweigh potential gain. The challenger is compelled—through deterrence, coercion, and compellence—to choose not to strike (pp. 16–17). Through tracing the literature on DT, Wilner explores different aspects of deterrence that, he argues, are relevant to the (primarily) religiously inspired non-state terrorist groups that dominate current security debates. Deterrence does not consist of punishment alone, he argues, but can involve denial of perceived benefits and “nonkinetic deterrents that rely on positive inducement and reassurance, denial and mitigation, and debate and delegitimization” (p. 30).

Chapter 3 seeks to “expand the logic” (p. 38) of these broad interpretations of deterrence to cover the contemporary terrorist threat. Tailored deterrents, the author argues, can be applied to a host of different roles, responsibilities, and “individual elements” (p. 43) that contribute to the terrorist threat and the values that terrorist groups and individuals hold dear, such as legitimacy, popularity, and social cohesiveness, in addition to tactical and strategic triumphs. Terrorism, according to this logic, can thus be deterred by punishment (targeting what terrorists value), denial (targeting what terrorists want), and delegitimization (targeting what terrorists believe).

How does this work in practice? Chapters 4 and 5 inject empirical evidence to support these theoretical assertions. These chapters constitute the mainstay of the book and delve into the detail of targeted killings and the way in which this tactic of warfare altered the behaviour of the Afghan Taliban. Wilner begins by separating targeting killings and assassinations, although the distinctions often seem hazy and subjective, before discussing the utility and hazards of targeting killing against terrorism at length. Of greater relevance to debates on the role of deterrence, he then explores the use of targeted killings as coercion: where a state’s limited use of targeted killings translates into a threat that is communicated clearly and which the challenger sees as credible, thus manipulating his behaviour. In a detailed dissection of Afghan Taliban attack patterns in the weeks immediately before and after four distinct targeted killings of Taliban commanders, Wilner finds that although targeted killing did not result in reduced violence overall, levels of sophistication and professionalism decreased significantly.

These empirical findings are woven into broader deliberations about the efficacy of deterrence against terrorism in the concluding chapter, where Wilner emphasises the need for tailoring coercive communication to limit particular manifestations of the terrorist threat. Again, the perceived efficacy of these measures rests on the assumption that terrorists are sufficiently rational to be able to respond to such types of communication.

The notion that terrorists are fundamentally rational has become widely accepted among scholars. A seminal study on the “root causes of terrorism” led by Tore Bjørgo (Root Causes of Terrorism: Myths, Reality and Ways Forward, 2006: 257), for example, concluded that “terrorists are not insane or irrational actors.” Whilst terrorists may not be mad, however (excepting some lone actors), scholars such as Max Abrahms (“What Terrorists Really Want: Terrorist Motives and Counterterrorism Strategy,” International Security, no 4, Spring 2008, pp. 78–105) have questioned the ability of terrorists to respond rationally (i.e. by maximising political utility) to coercive communication.

Notions of deterring terrorism also rest on central questions about what terrorists really want and seek to acomplish. After all, deterrence must focus on what is valued most in order to be effective. Wilner does explore, briefly, the extent to which physical security might not be at the top of the agenda of religiously-inspired terrorists and how non-violent efforts such as delegitimisation can expand the repertoire of deterrence in this context. Yet this leaves questions of agency and exploitation unanswered. Terrorism experts and policy makers have discussed the potential utility of counternarratives—closely related to Wilner’s notions of delegitimisation—for some time now, but questions remain about how these can be delieverd, what they would consist of, and how their impact could be measured. It seems even harder to envisage how the “threat” of such nonkinetic measures could be deployed directly as a form of deterrence. How does one deter, for example, against quests for rewards in the afterlife? This is not to say that detering such emotions and desires is impossible, but Wilner’s volume does not offer significantly new insights in this regard.

Adding delegitimisation and denial to the more conventional deterrence toolkit also risks expanding our understanding of deterrence to such a degree that its application in practice seems virtually indistuingishable from counterterrorism more broadly. The British CONTEST counterterrorism strategy, for example, explicitly incorporates coercion, punishment, tactical denial, persuasion, mitigation, delegitimization, and other measures that Wilner describes as forms of deterrence.

Incorporating both individuals and organisations as levels of analyses, of course, also complicates the study. As John Horgan has noted (e.g.: “Don't Ask Why People Join the Islamic State—Ask How,” Vice, September 10, 2014), it is hard to gauge an individual terrorist’s motivation as many are themselves unsure of what they are fighting for. Groups, such as Al-Qaeda in Iraq, may also be successfully deterred or defeated, but new organisations, such as ISIL, may still emerge from the broader movement in which groups are embedded. “Draining the swamp” through coercion, compellence, and deterrence is much harder.

Yet, Wilner recognises that deterrence is no panacea and explores in fascinating detail how behaviours were affected through campaigns of targeted killing in Afghanistan. His investigation of tactical deterrence wthin these scenarios is sound and valuable. His much more ambitious and broadstroak assessent of the applicability of deterrence against religious transnational terrorists raises more questions than answers but presents the reader with a throught-provoking, accessible, and informative discussion.