Underground Warfare, written by Daphné Richemond-Barak, an assistant professor at the Lauder School of Government, Diplomacy, and Strategy and senior researcher and head of the International Law Desk at the International Institute for Counter-Terrorism at the Interdisciplinary Center (IDC) Herzliya, Israel, tackles the ancient but insufficiently addressed phenomenon of tunnel warfare.

Before diving into the content, it is important to assure readers that it is unnecessary to have any prior knowledge of tunnels or their relevance in modern warfare. Having grown up in Texas, where my only exposure to tunnels was the occasional local news story about drug cartels or human traffickers using tunnels to infiltrate the U.S. southern border, I was pleasantly surprised that, in addition to finding references to the cartel-tunnels of my youth, I was able to quickly jump into the substance of this book. While tunnels may not be the subject of many readers’ day-to-day reflection, even those without any prior interest in underground warfare are encouraged to read this book in order to better understand more general international humanitarian law (IHL) and international law concepts.

When one thinks of what modern warfare involves, it is likely a person's initial instinct to think of cyber attacks, drones, satellites, and “killer robots.” Tunnels are far less likely to make that list, but Richemond-Barak's Underground Warfare may change that going forward. She has taken what may often be narrowly thought of as an “Israel-Palestine” problem and presented a quandary of significant historical interest as well as of immediate concern. While conflicts in the skies, sea, or cyberspace all have specifically tailored guidance, if not law, Richemond-Barak makes a convincing argument that underground warfareFootnote ¹ is just as widespread and diverse, and pushes states to be more proactive in developing norms for this neglected area.

Richemond-Barak presents three overarching arguments throughout her book: (1) underground warfare “has re-emerged on contemporary battlefields” (p. xvi); (2) underground warfare raises unique operational and legal issues; and (3) “proximity between tunnels and civilians” will complicate the application of existing “core legal principles” (id.).

To support these three suppositions, Richemond-Barak's first chapter guides the reader through the history of the use of tunnels in warfare, starting in World War I and continuing through World War II, the conflicts in North Korea, Vietnam, Afghanistan, Libya, Mali, and Israel, and finally, the use of tunnels by ISIS in Iraq. This historical overview provides an excellent factual backdrop for the remainder of the book, which covers both the technical aspects of underground warfare, as well as various legal and operational considerations.

The second chapter explores the creation, use, and typology of tunnels, as well as how they are likely to be used in the future. The third chapter considers how sovereignty in domestic and international law is applied in the context of air, sea, and space. This chapter also provides a number of examples of “peacetime tunnels”—such as those used for drug or people smuggling—which is useful for better contextualizing Chapter 4, which provides a brief overview of possible applicable legal frameworks, including jus ad bellum, jus in bello, human rights law, the law of occupation, and jus post bellum. In this chapter, Richemond-Barak also advocates for states to develop a strategy for addressing issues relating to tunnels, regardless of whether they deal frequently with the phenomenon.

The next three chapters delve deeper into the specific legal challenges of underground warfare. Chapter 5 attempts to apply jus ad bellum to the context of tunnel warfare, while Chapter 6 addresses the rules of jus in belloFootnote ² as it applies to underground hostilities (or attacks against underground facilities). In her final substantive chapter, Richemond-Barak explores the legal and humanitarian challenges raised by underground warfare in urban areas.

The way Richemond-Barak has structured the book lends itself well to the topic. By first guiding the reader through the historical context of tunnels, the complex situation of underground warfare becomes quickly apparent. But Richemond-Barak also excels at making these complexities accessible to those less familiar with underground warfare. The following discussion examines a selection of the more interesting or controversial issues that she analyzes.

This is obviously not the first publication to attempt to provide “guidelines” for how international law should apply to a particular type of warfare. Although different in scope and structure to such expert guidance tools as The Tallinn Manual on the International Law Applicable to Cyber Warfare (Tallinn 1.0),Footnote ³The Tallinn Manual 2.0 on the International Law Applicable to Cyber Operations (Tallinn 2.0),Footnote ⁴ the San Remo Manual on International Law Applicable to Armed Conflicts at Sea,Footnote ⁵ or the HPCR Manual on International Law Applicable to Air and Missile Warfare,Footnote ⁶ Richemond-Barak has produced not only an academic tome of note on this discreet form of warfare, but has also provided pragmatic guidance and interpretations reminiscent of the abovementioned manuals. The strength of manuals like Tallinn 1.0 is that they represent the hard-won consensus of a large group of experts, which is obviously lacking in a single-author publication. Nonetheless, Richemond-Barak has managed to capture a breadth of views and recommendations that will serve the international community well unless and until such a time as a similar international group of experts gathers to consider the many difficult questions raised by Underground Warfare. This also raises the question of how such a treatise will be used by states, who may place a greater weight on guidelines produced by a group of (especially governmental) experts rather than the stand-alone opinion of one author.

In this reviewer's view, the comparison of tunnel warfare to that of cyber warfare is the most apt, as both air and space warfare are almost exclusively the domain of state or state-like groups, but cyber tactics are available to every individual, group, and government actor, just as tunnels are. A truly sophisticated cyber attack, such as Stuxnet, may require state resources, but less sophisticated cyber attacks may still cause significant disruption or damage, in cyber space or the “real world.” This mirrors tunnel-building, as the kinds of fortified/electrified tunnels built by the North Koreans to enable thousands of troops to enter South Korea are of a design that nonstate actors would be unlikely to match, but the majority of tunnels could easily be engineered by nonstate armed groups.

In addition to its availability to nonstate actors, cyber warfare is also an interesting comparator due to the dual-use predicament and the “unknown” factor. The “unknown” factor refers both to attribution (i.e., who built the tunnel or carried out an underground attack) as well as “knowing where the enemy hides, … assessing the extent of its destructive capabilities and localizing the threat” (p. 185). Cyber networks, like tunnels, tend to be equally useful for civilian and military purposes, but having sufficient information about their design and use makes legal principles like distinction incredibly difficult to apply in practice. As such, this review will primarily look at Underground Warfare through the lens of cyber warfare. This comparison will hopefully allow readers to simultaneously improve their understanding of the dilemmas posed by the application of international law to both tunnel and cyber warfare.

Richemond-Barak has provided an excellent framework for how the principle of sovereignty and jus ad bellum might apply to tunnel warfare. She asserts from the outset that territorial sovereignty “constitutes a fundamental right of the state under international law” (p. 60). While she acknowledges the limitations to this right, it is important to note that the underlying concept of territorial sovereignty is not settled law. Tallinn 2.0 ultimately agreed with Richemond-Barak's assertion,Footnote ⁷ but it acknowledges the explicit disagreement of some experts—including the U.S. government expert who took the position that there is no blanket international rule of territorial sovereignty, but only specific rules, such as the prohibition against the use of force by states against the territory of other states, as set forth in Article 2(4) of the UN Charter,Footnote ⁸ or the rules enshrining a state's right to natural underground resources in the example used by Richemond-Barak. If the Tallinn/Richemond-Barak approach is followed, then any encroachment on a state's territory—even if not rising to a use of force—would be an international violation of state sovereignty, even if the encroachment were merely to explore a cross-border underground resource like oil or, in the case of cyber, for example, “if an agent of one State uses a USB flash drive to introduce malware into cyber infrastructure located in another State.”Footnote ⁹ It is certainly worth noting that the sovereignty debate in the context of cyber warfare and the underground tends to emphasize the importance of territorial sovereignty, whereas state practice writ large would appear to lean toward a more cautious approach, recognizing that some breaches of sovereignty are so minor that it may be better to ignore them in order to presumably avoid unnecessary confrontations.

The prohibition on the use of force, and the measures available to the defending state, are particularly difficult with respect to both underground and cyber operations, due to the inherent factor of the “unknown,” and this is only amplified by the presence of nonstate actors. Overall, Richemond-Barak takes a very pragmatic stance toward jus ad bellum by encouraging states to react to cross-border incursions of tunnels onto their territory as including a “spectrum of options” (p. 157). This spectrum would rarely include the full-fledged invocation of self-defense against an armed attack except where the state has already been attacked through the use of a cross-border tunnel and the attack itself meets the criteria for an armed attack. Richemond-Barak is of the view that the existence of multiple tunnels which have not yet crossed the border, coupled with such factors as the “the past relationship with the actor and the proximity to civilians” might also give rise to a claim of anticipatory self-defense, although these criteria would likely be more relevant to the physical realm of tunnels rather than to the virtual world. On the other hand, the existence of a “single tunnel that has not yet crossed the border” or a lack of other circumstantial factors, such as proximity to civilians, would not permit this claim but would rather allow for a claim that there has been an unlawful use of force against the victim state (p. 158). In all of these cases, except a single tunnel that has not yet crossed a border, Richemond-Barak argues in favor of using the principle of necessity to permit a limited response to neutralize a tunnel or tunnels that have crossed or are likely to cross the border of the victim state in keeping with the requirement that “the peril be grave and imminent” (p. 159). Necessity permits the state to protect its immediate interests without escalating the situation, as would an invocation of self-defense, which would then open the door to more serious countermeasures.

Tallinn 2.0 and Richemond-Barak explore the obligation of “host” states to prevent cross-border incursions by a nonstate actor through cyber warfare or tunnel networks under the principle of due diligence. For example, Rule 6 of Tallinn 2.0 contends that “a State must exercise due diligence in not allowing its territory, or territory or cyber infrastructure under its governmental control, to be used for cyber operations that affect the rights of, and produce serious adverse consequences for, other States.”Footnote ¹⁰ Likewise, Richemond-Barak posits that a host state will likely be responsible for tunnels dug by nonstate actors when such activities are “predictable” due to past tunnel digging, or if the tunnel's location is such that the host state would be expected to know about its existence (p. 122). Here, the cyber analogy may be somewhat weaker, as it is comparably easier to spot physical digging of a tunnel than it is to spot cyber intrusions.

Richemond-Barak argues that the mere existence of a tunnel that may cross a border is a threat of force, whereas a completed cross-border tunnel is both a threat and a use of force (p. 143). If one takes a computer server, as a counterexample, however, under this view would the building or completion of that server amount to a use of force, if it is known that the server will be used by a state to carry out cyber attacks? This will always be more complex with cyber attacks, as servers are dual use, whereas tunnels are unlikely to have a lawful purpose near the border. Nonetheless, it is worth looking at how cyber experts have addressed this issue. Tallinn 1.0 looks at several factors to determine whether a use of force has occurred, including severity of physical harm, immediacy, measurability of effects, and the military character of the action.Footnote ¹¹ Richemond-Barak argues that these factors can equally be applied to the building of tunnels, but here is where an overreliance on the analogy of cyber warfare to tunnels may become problematic. The inherent danger in having a low threshold in finding that a use of force has occurred is found in the responses that are permitted. For example, Richemond-Barak asserts that a state may use a plea of necessity to destroy a cross-border tunnel on another's state's territory (provided other requirements, such as imminent peril, are met) even if the attacking state has no idea who made the tunnel or why. She compares this to the Tallinn 1.0 interpretation of necessity which would allow states to take defensive cyber measures, like disabling cyber infrastructure, when there is a cyber attack of unknown origin “even if doing so affects cyber-systems in other States.”Footnote ¹² Regarding either the building of cyber or tunnel networks as a potential use of force, there is a great risk for escalation by states. The response to the building of a cross-border tunnel will inevitably result in physical harm, as Richemond-Barak notes when discussing the neutralization and destruction of tunnels, and there is a fairly high chance of that harm being to civilians or civilian infrastructure, particularly if the tunnels are found in urban areas near a border. On the other hand, the cyber measures envisaged by Tallinn are far less likely to result in any kind of physical harm, even if they do have an impact on the civilian population.

Turning to the book's analysis of jus in bello, Richemond-Barak addresses a wide range of IHL rules in relation to tunnel warfare. This section will focus on two key issues that are of interest for cyber warfare as well: (1) tunnels as war-sustaining objects; and (2) the concept of “knowledge” under IHL.

To understand the debate on war-sustaining objects, one must define what constitutes a military objective, as only military objectives may be directly targeted under IHL. Military objectives are defined by Article 52(2) of Additional Protocol I as “limited to those objects which by their nature, location, purpose or use make an effective contribution to military action and whose total or partial destruction, capture or neutralization, in the circumstances ruling at the time, offers a definite military advantage.”Footnote ¹³

Richemond-Barak explores tunnels in relation to the issue of war-sustaining objects, or “objects that make an indirect contribution to military action” (p. 182, emphasis added). The theory of war-sustaining objects is controversial as it allows for the targeting of objects that do not otherwise meet the definition of a military objective because their destruction, neutralization, or capture is not sufficiently directly linked to military action or “war-fighting” activities. Richemond-Barak alludes to the example of smuggling tunnels which have no direct connection to the military, but which generate funds that “fuel the war effort,” and she finds that they may be targetable as war-sustaining objects (id.). This is the same justification used by the U.S. military to justify the targeting of oil fields in Iraq that had generated revenue for ISIS.Footnote ¹⁴ The United States has long held that the “effective contribution” does not have to be to “military action” but rather to “war-sustaining” activities.Footnote ¹⁵

Tallinn 1.0 addressed the notion of war-sustaining targets. The majority of experts found that the use of cyber means to attack an economic target such as the “enemy State's oil export industry” would not be permissible “on the ground that the connection between war-sustaining activities and military action was too remote.”Footnote ¹⁶Tallinn 1.0 used the example of a cyber attack on the “civilian business sector,” which would likely give the attacker an economic advantage, but “would not necessarily result in military advantage in the sense of affecting … military operations in a relatively direct fashion.”Footnote ¹⁷ This rejection of the notion of direct targeting of war-sustaining objects that do not otherwise meet the criteria for military objects was confirmed by the International Law Association Study Group, which found that the U.S. positionFootnote ¹⁸ “has no basis in the law as it stands today and should be clearly rejected.”Footnote ¹⁹ While Richemond-Barak's example of smuggling tunnels could in some circumstances meet the criteria of an effective contribution to military action, such as smuggling weapons, in many cases, the neutralization of a tunnel used only for smuggling goods—even illegal contraband—will be too remote. That is not to say states are left without any recourse. Economic sanctions, arrests, or the other measures suggested by Richemond-Barak for neutralizing tunnels (e.g., cement, smoke) that do not rise to the level of an “attack” under IHL may still be used.

The second jus in bello topic highlighted in Underground Warfare is that of the concept of “knowledge” under IHL. Various IHL rules “governing the conduct of hostilities require the commander to possess both knowledge and information” (p. 187). Yet IHL is not explicit about the amount of information or knowledge that is required, but rather imposes a duty to do “everything feasible” to verify the information available at the time.Footnote ²⁰ This issue may arise with respect to attribution of an attack, knowledge of whether an attack on a segment of a tunnel will offer a definite military advantage, and knowledge of whether a civilian is acting as a human shield, to name a few.

Both tunnels and cyber operations challenge the commander's ability to verify information and raise the question of whether this frees states from their obligation to gather information or whether it requires them to suspend or cancel attacks when sufficient information cannot be obtained. Tallinn 1.0 tackles this issue by placing a greater burden on commanders “during the planning and decision phases of the attack.”Footnote ²¹ While Richemond-Barak acknowledges that the “unknown factor” in cyber warfare is greater than that of tunnel warfare, she nonetheless applies the Tallinn view on precautions to tunnel warfare. Whereas the nature of the threat posed by a tunnel to the defending states or the surrounding civilian population may be unknowable, the consequences of physically destroying that tunnel are far easier to ascertain than the consequences of releasing a code into cyberspace, especially as most cyber attacks are not currently used to cause physical harm. Therefore, the knowledge requirement for attacking tunnels should not be unnecessarily lowered based on a comparison to cyber norms.

Before concluding, I wanted to note Richemond-Barak's suggestion that “IHL should … prohibit the simultaneous use of underground structures for civilian and military purposes.” This seems implausible for the same reason that cyber networks cannot be completely separated into “civilian” and “military” networks. While it would prevent such tragedies as the bombing of the Al Firdos bunker in Iraq which was mistakenly targeted in 1991 by Coalition forces who were unaware that the bunker was used as a civilian shelter at night (p. 224), it would seem to ignore the realities of urban warfare, in particular where civilians may have very few options when seeking safety from aerial bombardments or other attacks. Richemond-Barak does acknowledge that civilians may not always have alternatives, but she nonetheless appears to place the onus on civilians to make sure they are not in harm's way, which is not where the law places this burden.

For this less-than-tech-savvy reviewer, Underground Warfare greatly enhanced my own grasp of cyber warfare by providing a more tangible framework (i.e., tunnels) in which to analyze these complex legal issues. Occasionally, the emphasis on the uniqueness of tunnel warfare detracted from Richemond-Barak's otherwise excellent analysis. And while Richemond-Barak's focus on developing a military strategy to address tunnels is appreciated, as violations of international law often seem to be the result of poor planning, her recommendations on the use of specific military tactics would be more convincing in the context of a determination by experts. However, the most important takeaway from Underground Warfare is the impact of the “unknown” on compliance with international norms. If we there is any hope of not stretching existing law beyond its breaking point, this aspect should be taken more seriously, not only in relation to tunnel and cyber warfare, but with respect to all emerging technologies and modern methods of warfare.