When we make law, we create categories; when we interpret law, we assign actions, actors, and events to the categories we have created. For law to be effective, we need reasonable clarity and consensus about the contents and value of our categories: the concept of “theft,” for instance, makes sense only if there is some shared understanding of the concepts of “property” and “ownership.”

This is equally true of the legal framework governing conflict and coercion. Both international and US law take as a basic premise the notion that it is possible, important, and reasonably straightforward to distinguish between war and peace, emergencies and normality, foreign and domestic, public and private, and so on. We have elaborate rules governing the conduct of “parties” to “armed conflicts”; we subdivide people into “combatants” and “civilians”; we speak of “force,” “self-defense,” “armed attacks,” and actions falling short of armed attacks; we distinguish between areas with “active hostilities” and areas without such hostilities, between “internal,” “international,” and “noninternational” armed conflicts, and between civilians who are “directly participating” in hostilities and those who are not.

A great deal hinges on our ability to apply these categories in a coherent, predictable, and principled way. A combatant in an armed conflict can lawfully be targeted by the armed forces of an opposing party, but it’s a war crime to target a civilian who is not directly participating in hostilities. A state can lawfully use unilateral force inside the borders of a nonconsenting state when defending against an actual or imminent armed attack, but other unilateral uses of force inside nonconsenting states violate international law and subject the aggressor state to the possibility of international sanction. Combatants and civilians directly participating in hostilities can be detained by opposing parties for the sole purpose of removing them from the battlefield, without benefit of attorneys or trials; absent an armed conflict, states can detain individuals only after providing them with due process.

On the margins, these legal categories have always been contested. Proxy wars, mercenaries, insurgencies, and terrorist groups have always posed challenges to the post–World War II legal framework, and both states and nonstate actors have always sought nonkinetic means to influence and coerce others through the use of economic pressure, propaganda, espionage, and so forth. In this sense, what we today call “gray-zone” conflicts are nothing new—and efforts by actors to exploit gaps and ambiguities in the law are also nothing new.

But in recent decades, technological changes have increasingly made gray-zone challenges the norm, rather than the exception. Our increasing global interconnectedness has created new vulnerabilities for states and communities, as has our increasing dependence on the internet and other forms of electronic communication. Nonstate actors can increasingly compete with states when it comes to using physical force to cause large-scale death and physical injury, and the use of physical force itself has more and more competition: war, wrote Clausewitz, is “an act of violence to compel our opponent to fulfill our will,” but our increased interconnectedness has created new means for clever actors—whether states or individuals—to achieve war’s traditional ends. Cyberattacks can cause massive political and economic disruption; propaganda campaigns can spread at warp speed through social media channels.

As a result, it’s grown progressively more difficult to apply basic legal concepts relating to war and the use of force in a coherent way. The post–World War II international legal framework assumes a state-centric world in which organized, conventional militaries vie for control of physical terrain. But in a cyberwar or a war on terrorism, there can be no boundaries in time or physical space: we can’t point to the battlefield on a map or articulate circumstances in which such a war might end. In the gray zone, we don’t know what counts as “armed conflict” or “the use of force,” and physical terrain may be irrelevant in battles for political influence.

In the gray zone, we’re no longer even sure what counts as a weapon: A hijacked passenger plane? A line of computer code? A fake news story, artfully disseminated? Often, we also can’t define the enemy—decentralized, transnational, nonstate actors morph unpredictably, and states have developed new means of evading traditional attribution methods. Similarly, we struggle to tell the difference between “civilians” and “combatants.” What counts as a protected civilian object in cyberspace? When can a hacker, a financier, or a propagandist be considered a combatant? When, if ever, is it lawful for a state to respond to a nonkinetic “attack” using conventional military force?

Gray-zone actors deliberately exploit these uncertainties to evade legal responsibility and minimize military, economic, and political consequences. Chinese government–sponsored hackers steal sensitive US military information, for instance, but attribution difficulties—and the unacceptable costs of overt military confrontation—keep the United States from responding decisively; in the South China Sea, Chinese “coast guard” ships patrol the waters off new man-made Chinese “islands,” and “civilian” Chinese fishing vessels deliberately cause near collisions with US military vessels to keep them from getting too close to sites or exercises considered sensitive by the Chinese.

Russia too has proven adept at gray-zone activities: while an overt Russian invasion of Ukraine might have triggered a direct military confrontation with the United States and other NATO members, Russian cyberattacks, manipulation of videos and photos, and “little green men” were just ambiguous enough to sow confusion about the role of the Russian government, thus minimizing the likelihood of a prompt and decisive international response. More recently, Russia has engaged in a concerted, covert effort to influence US elections; here too, however, its methods have been indirect, and although US intelligence agencies assert that there can be no doubt about Russian interference, the White House has been unwilling to offer an unequivocal response.

Much recent discussion of gray-zone conflict has focused on US difficulties in responding to gray-zone challenges, whether those challenges stem from states such as Russia, China, Iran, and North Korea or from nonstate entities such as ISIS. Despite its overwhelming conventional military superiority and its technological sophistication, the United States has had trouble responding effectively to gray-zone threats: as Philip Kapusta noted in a 2015 article,

Decisive actions in the gray zone are far easier to carry out by authoritarian or centralized decision-making structures than by democratic, consensus-building governments and coalitions. . . . Gray zone challenges tend to involve multiple instruments of power simultaneously, and unity of command is helpful in achieving rapid and effective results. . . . The net effect in democracies is to create intense bureaucratic friction arising from our own organizing principles, resulting in strategic and operational rigidity.

Nonetheless, in some ways the United States has also been a gray-zone pioneer. After 9/11, for instance, the United States rapidly developed legal theories and tactics designed to exploit gaps and ambiguities in the international legal framework governing armed conflict and the use of force. In 2001 and 2002, for instance, the United States argued that the Geneva Conventions (including Common Article 3) were wholly inapplicable to both al-Qaeda and Taliban detainees, thus making it lawful for the United States to hold detainees in secret. Similarly, despite being a signatory to the UN Convention Against Torture, US government lawyers argued that “enhanced” interrogation methods such as waterboarding did not constitute torture, and since Geneva protections were deemed not to apply, the United States took the view that there was also no legal bar to humiliating or degrading treatment of detainees. Meanwhile, the United States utilized “extraordinary rendition” and secret agreements with partner governments to spirit detainees away from the prying eyes of journalists and NGOs.

As with recent Russian and Chinese actions, US activities in the years immediately following the 9/11 attacks were carefully calibrated to bypass inconvenient international legal restrictions while remaining superficially compliant with the law—or, at any rate, just compliant enough, on the level or rhetoric, to minimize international opposition.

Recent US targeted counterterrorism strikes can similarly be viewed as gray-zone activities. By insisting that targeted US strikes in sovereign states are only made when the state at issue consents or when such strikes are necessary in self defense, the United States deflects claims that such strikes constitute unlawful uses of force, violating UN Charter provisions and long-standing principles of sovereign nonintervention. But since the United States has for the most part declined to formally acknowledge particular strikes, identify targets, or specify the factual or legal basis for particular strikes, critics can neither definitively prove US involvement nor critique specific US actions. (Have all US targets been individuals who could plausibly be viewed as combatants in an armed conflict, or civilians directly participating in hostilities? Alternatively, could the targets of such strikes plausibly be viewed as posing an imminent threat of armed attack to the United States? Given the secrecy and lack of formal acknowledgement, no one can say for sure.) In effect, the lack of certainty about US actions or motivations makes it impossible for external actors to determine the lawfulness of US strikes.

In the face of this ambiguity, the response to US counterterrorism strikes from foreign governments—including US allies generally committed to more traditional and formalist interpretations of international law—has been muted; although representatives of allied governments frequently express private concerns in off-the-record settings, few have been willing to openly condemn US strikes.

In the near term, this has benefited the United States, allowing it more freedom of action, just as gray-zone activities have offered near-term benefits to Russia, China, and other states. Whether such counterterrorism strikes help the United States achieve its longer-term political objectives is a separate question.

State gray-zone activities both exploit and create legal ambiguity, and collectively, state gray-zone activities represent a significant challenge to the international rule of law. This is not because such gray-zone activities violate international law: ironically, they might be less destabilizing, from a rule-of-law perspective, if they could be easily categorized as blatant instances of rule breaking. The gray-zone activities of powerful states challenge the international rule of law precisely because they defy straightforward legal categorization. In a sense, state gray-zone activities constitute a serious and sustained assault on the once widely accepted meaning of core international legal concepts and categories such as “self-defense,” “armed attack,” “imminence,” “combatant,” “civilian,” “armed conflict,” and “hostilities.”

Like all law, the international law relating to armed conflicts and the use of force has always been somewhat vague and ambiguous; and, to a degree, this can be seen as a virtue rather than a vice in a system that lacks a judiciary and a reliable enforcement mechanism. Up to a point, legal vagueness and ambiguity give states face-saving ways to avoid direct conflict, enabling them to “look the other way” if a particular state occasionally engages in challenging but not manifestly illegal behavior. Vagueness and ambiguity can also sometimes offer an efficient way for consensus-based changes in the law: for instance, amending the language of international treaties might be cumbersome or impossible, but some degree of vagueness and ambiguity in treaty language can permit shared interpretations to be modified over time, thus providing the community of states with a relatively simple “backdoor” means of changing the effect of a treaty.

Beyond a certain point, however, vagueness and ambiguity are crippling. When key international law concepts and categories lose all fixed meaning, consensus breaks down about how to evaluate state behavior; and although legal rules may continue to exist on paper, they no longer do much to ensure that states will behave in a predictable, nonarbitrary fashion. Among other things, this can increase instability: when no one can be sure what level of incremental gray-zone activity will trigger a decisive response from another state, the risk of accidental escalation goes up.

When one or more powerful states engage in gray-zone activities that issue de facto challenges to the generally accepted meaning of core legal concepts, other states face a choice. They can accept the “new” interpretations of international law, in which case—if a sufficient number of states take the same route—international law will quietly change (e.g., the international community could choose to accept China’s man-made islands as a legitimate means of extending its territorial waters, opening the way for other littoral states to engage in similar tactics). Alternatively, they can take the opposite tack, directly confronting those states seeking to reinterpret the law and demanding fidelity to previously shared interpretations. This route is risky: if it succeeds, legal stability is restored, but if it fails, there is an increased chance that legal disputes will escalate into open conflict.

Finally, states dismayed by new interpretations of once-fixed legal concepts can take a middle ground, quietly questioning “new” interpretations of the law while reaffirming their own interpretations. This route reduces the likelihood of escalation and overt conflict, but by enabling disparate legal interpretations to coexist without any obvious means of reconciling them, it can also prolong or increase legal uncertainty.

At its most fundamental level, the rule of law is concerned with constraining and ordering power and violence. In the international domain, where there is no authoritative judiciary or legislature capable of rapidly clarifying the law, legality (or illegality) must still be inferred from state actions and commentary (in legal terms, from practice and opinion juris). But state gray-zone activities risk leaving international law on the use of armed force not merely vague or ambiguous but effectively indeterminate, eroding law’s value both as a predictor of state conduct and a means of holding states accountable.

Of course, the proliferation of gray-zone activities could also spur legal and institutional innovation. If all or most states come to accept a new interpretation of key terms and concepts relating to the use of force, the international law on the use of force will evolve. If states cannot agree on how to interpret key concepts, the risk of conflict between states will go up, but the increased risk of conflict may itself trigger the creation of new dispute-resolution mechanisms (be they judicial or nonjudicial), which can in turn develop new authoritative interpretations of the law.

Looking ahead, how should the United States respond to the proliferation of gray-zone activities?

For more than half a century, the United States has been the primary guarantor of the post–World War II international order, but recent US actions in the gray zone have contributed to undermining many of the very norms the United States once helped promulgate—making it more difficult, in consequence, for the United States to credibly condemn even more questionable gray-zone actions by states such as Russia and China.

In the long run, the best way for the United States to mitigate and constrain the gray-zone activities of adversarial states will be to return to its traditional role as champion of a rules-based international order. As Michael Mazarr has argued,

The most important [US] answer to gray zone tactics is not to build a whole suite of counter-capabilities—fishing fleets, battalions of “little green men,” propaganda campaigns. Some of that would be useful, especially for the United States, whose military is traditionally oriented toward major combat operations. But [adversary] gray zone campaigns are most likely to fail when they cannot sneak under the radar of the international system. The most important and ultimately effective response will therefore be to reaffirm and strengthen the norms, rules, and institutions of the international order in ways that render these campaigns even more provocative and self-defeating than they already are.

This does not mean that the United States should simply revert to championing traditional, formalist interpretations of international law. On the contrary: the proliferation of gray-zone challenges is testament to the weaknesses of the post-war legal order, and the United States is not wrong to argue that the binary legal categories we have inherited—war/peace, national/international, public/private—are no longer adequate in our complex, multipolar, and interconnected world. Our challenge, in the years to come, will be to envision and help create a more equitable and inclusive world order, one with robust collective problem-solving and dispute-resolution mechanisms. This will require a commitment to working collaboratively both with allies and with rising powers and responsible nonstate actors to develop appropriate and transparent rules for the space between war and peace: rules for cyber conduct, for instance; rules to ensure accountability in state uses of force against nonstate actors; rules concerning the use of contractors and other private actors; and so on.

This will be a monumental undertaking, on the scale of the post–World War II reconfiguration of the international order. In the twentieth century, it took two catastrophic global conflicts to motivate the United States to fundamentally rethink international rules and institutions. Let’s hope it won’t take another catastrophe to spur the innovative thinking the twenty-first-century United States so desperately needs.


Rosa Brooks is the Associate Dean for Graduate Programs and Professor of Law at Georgetown University Law Center and an Adjunct Scholar with the Modern War Institute. She also writes a column for Foreign Policy and serves as a Senior Fellow in the New America/ASU Future of War Program. Her most recent book is How Everything Became War and the Military Became Everything (Simon and Schuster, 2016).

This is an unofficial expression of opinion; the views expressed are those of the author and not necessarily those of West Point, the Department of the Army, the Department of Defense, or any agency of the US government.