International Law Espionage in International Law
François Dubuisson, Agatha Verdebout
  • LAST REVIEWED: 24 February 2021
  • LAST MODIFIED: 25 September 2018
  • DOI: 10.1093/obo/9780199796953-0173


International espionage consists of the access, on behalf of a state, to information that is held by another state and considered as confidential or strategic, in the military, security, or economic field. This classical conception has evolved to also include surveillance programs implemented by intelligence agencies toward individuals as well as company-to-company industrial espionage. While for a long time espionage essentially took place in the physical space, in the 21st century, it primarily takes place in the cyberspace. It should also be noted that, against common assumptions, the notion exclusively refers to the gathering of information and does not refer to covert operations carried out by secret services more generally. Although such operations are a very old and common practice, it is paradoxically not regulated by a single and coherent legal regime under international law. Only the status of spies in times of war is, in fact, subject to specific international rules. Moreover, until quite recently, the legal issues raised by espionage have largely been ignored by the doctrine. It was only in the context of the Cold War—in light of various incidents that opposed the United States and the USSR—that authors started to take a real interest in interstate spying activities on land and sea, and in air and space. The analyses provided then focused on general international law and on the principle of state sovereignty. American authors, most of whom held public office, dominated the field. This often resulted in a literature that defended the legality of intelligence-gathering activities as conducted by US agencies. That being said, more critical writings also exist. Recent studies, in particular, have started to question the compatibility of surveillance practices with human rights law. After the 9/11 attacks, the issue of espionage has, indeed, gained new momentum due to the implementation of new augmented surveillance programs for the purpose of fighting terrorism. The development of new technologies and communication media has, likewise, heightened possibilities for mass surveillance. In June 2013, the revelations made by Edward Snowden, a former agent of the National Secret Agency (NSA), concerning the United States’ intelligence programs reinvigorated legal and political debate around espionage activities by states. In legal scholarship, discussions have mainly focused on the legality of espionage under international human rights law. Forms of espionage have become more diverse and sophisticated, involving a wide array of practices and actors. Because of this diversity, and of the absence of a single, general legal regime under international law, the problems raised by espionage require the examination of a different set of rules: sovereignty; nonintervention; use of force; sea, air, and space law; human rights; international economic law; international criminal law; etc. Generally speaking, however, two types of approaches can be found in the doctrine. The first consists in a global approach, which examines espionage in a transversal way and in its various dimensions (see General Overviews). The second, on the other hand, is a sectorial approach that tends to analyze specific espionage activities in light of a particular set of rules (see the other sections).

General Overviews

A certain number of publications give a general overview of the various questions raised by espionage activities under international law. The main question is whether such activities are, in and of themself, legal. Three different positions can be found in doctrine. According to the majority opinion, there is no general prohibitive rule against espionage under international law. Cohen-Jonathan and Kovar 1960 thus already noted that state practice showed that there was no clear rule forbidding espionage between states, even in times of peace. More recently, Kish and Turns 1995, Lafouasse 2012, and Chesterman 2006 came to the same conclusion, emphasizing that the legality of espionage should, as a result, be assessed in light of sectorial rules (whether human rights, humanitarian law, or also diplomatic law). By contrast, according to a minority, espionage is prohibited by general international law. Wright 1962 (cited under Espionage and Sovereignty), indeed, argues that these activities constitute an infringement of sovereignty and, as such, a violation of the principle of nonintervention. The third view holds that espionage stands in a grey area of international law, as it is neither explicitly forbidden nor clearly authorized. Yoo and Sulmasy 2007 argues that this is because states do not have an interest in regulating espionage, as doing so would hinder their capacity to protect their national security. Along the same lines, Baker 2003 contends that states actually see espionage as a means to foster international cooperation. These doctrinal divergences reflect the particular secretive nature of the state practice. Whether as perpetrators or as victims, states are, in fact, rarely inclined to publicly communicate on the matter. This certainly explains most of the uncertainties that, in the early 21st century, continue to surround the international legal regime of espionage. In face of these uncertainties, some authors have thus sought to establish abstract models for evaluation. Deeks 2016 thus advocates for a “pragmatic” system that would determine the legality of acts of espionage depending on the interests at stake. In response, Forcese 2016 warns against the perverse side effects that such approaches could have on the overall international legal system.

  • Baker, Christopher D. “Tolerance of International Espionage: A Functional Approach.” American University International Law Review 19 (2003): 1091–1113.

    This article takes the view that international law neither permits nor prohibits espionage. According to the author, this conclusion can best be explained if espionage is considered as a functional tool promoting international cooperation. States would be more willing to cooperate in a system where they could, later on, monitor the behavior of their foreign partners.

  • Chesterman, Simon. “The Spy Who Came in from the Cold War: Intelligence and International Law.” Michigan Journal of International Law 27 (2006): 1071–1130.

    This study examines the international legal regime regulating the gathering of secret intelligence as well as the rules controlling the use of such information once obtained. It is a thorough article whose originality lies in the analysis of the legal issues raised by the use of information obtained through espionage before different international bodies, like the UN Security Council or the International Criminal Court.

  • Cohen-Jonathan, Gérard, and Robert Kovar. “L’espionnage en temps de paix.” Annuaire Français de Droit International 6.1 (1960): 239–255.

    DOI: 10.3406/afdi.1960.903

    One of the first general studies ever written on the topic. The article sets out the elements characterizing international espionage (object, intention, victim, beneficiary), and then it proceeds to the analysis of its legality in peacetime. In light of state practice, the authors conclude that there is no clear international rule prohibiting espionage, spying activities being generally regarded as falling within the category of “unfriendly acts” between states.

  • Deeks, Ashley. “Pragmatism and Principles: Intelligence Agencies and International Law.” Virginia Law Review 102 (2016): 599–685.

    Starting from the observation that the status of espionage under international law is uncertain, this article proposes a model for assessing the legality of espionage acts. It argues that there is a need to reconcile imperatives of pragmatism and realism with principles and legal formalism. More concretely, it suggests balancing the interests in presence on the basis of four criteria: risk of harm or error, nature of the target, applicable rules, and nature of the operation.

  • Forcese, Craig. “Confronting and Adapting: Intelligence Agencies and International Law.” Virginia Law Review 102 (2016): 67–84.

    This is a response to Deeks 2016. Claiming that the uncertainty of international law relative to espionage should not be overstated as a result of sectorial rules, the author warns against the temptation of tempering legal formalism in the name of pragmatism. Deeks’s sliding-scale model, he fears, would result in an overall weakening of the protection offered by the existing international legal frameworks.

  • Kish, John, and David Turns. International Law and Espionage. The Hague: Martinus Nijhoff, 1995.

    The first (and only) monograph written in English on the legal status of espionage in international law. The authors examine espionage from four different perspectives: human rights, diplomatic law, territory, and the law of war. The book shows that espionage is not regulated by a single legal regime, but rather by several different ones, depending on the context in which the question of the legality of espionage arises.

  • Lafouasse, Fabien. L’espionnage dans le droit international. Paris: Nouveau Monde Éditions, 2012.

    DOI: 10.14375/NP.9782847366549

    This book is the most comprehensive and thorough study of international law rules regulating espionage. After a detailed analysis of practice and of the relevant rules of law, the author argues that there is a summa divisio between, on the one hand, acts of espionage permitted under international law (e.g., under humanitarian law), and on the other, those that violate international law (e.g., when they breach territorial sovereignty or rules of diplomatic law).

  • Yoo, John, and Glenn Sulmasy. “Counterintuitive: Intelligence Operations and International Law.” Michigan Journal of International Law 28 (2007): 625–638.

    The central question of this article is whether international law is required to deal with states’ covert intelligence-gathering activities during peacetime. The authors conclude that it is not in the interest of states to regulate espionage activities, insofar as espionage is necessary for the defense of their security. The article illustrates an objectivist conception of international law, giving precedence to requirements of necessity in determining the content of the relevant international rules.

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