At a time when tangible success in the global war on terror seems increasingly elusive, and critics question not only its direct costs (both economic and in human lives) but also the indirect costs of resources diverted from other important concerns, Oded Löwenheim provides a timely explanation of the single-minded U.S. focus on Al Qaeda at the expense of other (and some would argue) equally if not more destructive actors in international society. The author argues that Al Qaeda, and terrorist groups in general, can be viewed as one among many subnational actors he designates as PATHs, “persistent agents of transnational harm,” and as such, they are usefully understood in terms of their impact (perceived or otherwise) on Great Power authority.

A central thesis of the rigorously argued Predators and Parasites is that Great Powers will respond aggressively to PATHs they view as “predatory” but less so to those they view as merely “parasitic.” Löwenheim maintains that authority, or “legitimized power that invests its bearer with special rights and duties,” is fundamental to Great Powers, and that the main role of a Great Power in this “structure of authority” is to regulate and institutionalize transborder violence (pp. 22–23). PATHs that seek to destroy or replace these structures of authority are predators; those that merely abuse them are parasites. Importantly, whether a PATH is a predator or parasite is not a function of economic costs; PATHs that cause significant financial damage to Great Powers may be treated with more tolerance than those that impose little or no material harm. Rather, it is those that provoke an “authority dilemma” by threatening (or seeming to threaten) the fundamental political/social/moral authority and existence of the Great Powers that cannot be tolerated. Löwenheim illustrates his argument through the examination of three case studies: 1) Spain's response to the Barbary pirates in the sixteenth and seventeenth centuries, 2) Great Britain's efforts against the Barbary pirates in the early nineteenth century, and 3) 9/11 and the U.S. war on terror.

The sixteenth and seventeenth centuries represented the height of the Barbary pirates' depredations. During this time, they imposed significant economic costs on Spain and other European powers in the form of ransoming, defense against attacks, and the loss of men, ships, and commerce. Spain initially chose to respond vigorously but then lost interest as attacks persisted. Löwenheim argues that this can be understood by the low “authority dilemma” faced by Spain. The notion of Great Power authority was still weak, and the norms of transborder violence not yet fully legitimized. Because no strong authority identity was threatened, Spain viewed the corsairs as essentially parasites or “pests,”

This changed in the nineteenth century. While the pirates were an insignificant economic threat to Great Britain, they represented a significant challenge to British moral and international authority. The Barbary corsairs were still engaged in slavery at a time when the British had undertaken its eradication. Furthermore, other states were beginning to look to Britain for protection. As such, the pirates' behavior presented a significant authority dilemma that could not be ignored. The result was aggressive action to eliminate this predatory challenge to British moral norms.

The U.S. response to 9/11 can be understood in similar terms. Löwenheim points out that Hezbollah's attacks in the early 1980s caused significant material harm, but elicited far less of a response from the United States than Libya's sponsorship of terrorism. In the former case, there was little challenge to U.S. authority, whereas Muammar Qaddafi in 1986 was believed to present a significant authority dilemma. Löwenheim's discussion of the U.S. response to Al Qaeda convincingly distinguishes between material harm per se and material harm that challenges Great Power authority. Al Qaeda is a threat because it defied U.S. superpower status in a manner completely opposed to the norms and institutions of international society. As such, it is a predator that must be opposed, resisted, and completely eradicated.

The book's central thesis could be strengthened if it addressed Great Power response (or nonresponse) to modern-day PATHs other than terrorists, specifically current maritime piracy. For example, viewing modern pirates as “parasites” offers a good explanation of U.S. nonengagement. Despite calls from the international community for assistance, the U.S. Navy has been reluctant to combat piracy. Only when it has been conjoined with “terrorism” (as is beginning to occur off the Horn of Africa) has the United States become more involved. It is interesting that when given the choice between countering a parasite versus a predator, the United States has opted to counter the predator, even when it has meant increasing the power of the parasite. When the Union of Islamic Courts (ICU) briefly came to power in Somalia, pirate attacks in the region dropped over 70%. However, after the U.S.-assisted Ethiopian overthrow of the ICU, piracy began to show signs of returning to its previous levels.

In addition, while Löwenheim's focuses on Great Powers, it could be useful to examine the role that smaller states play in dealing with PATHs that are largely ignored by the Great Powers. For example, most modern piracy is dealt with by smaller states; when these states choose to act—for example, Indonesia, Malaysia, and Singapore in the Straits of Malacca—they have effectively decreased the incidence of attacks. It would be interesting to explore whether smaller states apply the predatory/parasitic logic when confronting PATHs, and whether affected countries undertook antipiracy measures in Malacca because they viewed pirates as predators to their authority, rather than as parasites.

This said, Predators and Parasites is a rigorously argued discussion of how and when Great Powers engage persistent agents of transnational harm. By comparing Barbary piracy with Al Qaeda and other terrorist groups, Löwenheim has made a valuable contribution to a wide range of readers, including international relations theorists, maritime historians, and those concerned with “rogue” nonstate actors—be they terrorists, pirates, drug dealers, modern-day slavers, or any other organized criminal activity—in today's globalized society.