The legal assessment of sanctions is a complex endeavour. It combines many different areas of general international law with specific legal areas such as human rights or trade law, which apply only to certain domains. Furthermore, the question of whether or under which circumstances sanctions are lawful touches upon the foundations of international law. Unlike national law, international law lacks centralized enforcement mechanisms, making sanctions one of the few means of enforcing legal norms. As a result, sanctions can serve a legitimate legal claim, but their effectiveness is inseparably linked to the power of a state. Consequently, some states view them as leverage when enforcing international law, while others fear abuse of power and interference. Accordingly, states and the literature present very contradictory positions on the assessment of sanctions.
The legality of sanctions has been discussed with particular intensity over the last 30 years. Since the 1990s, the imposition of sanctions by the UN Security Council, which had previously rarely been effective due to the conditions of the Cold War, has increased exponentially. At the same time, individual states and groups of states have also used sanctions significantly more often than before, with the United States and the European Union being particularly frequent users of this instrument.
However, these sanction regimes, which often involved a comprehensive disruption of economic and financial relations, have proven to be ineffective. Moreover, comprehensive sanctions regularly led to devastating humanitarian crises. This was particularly evident in the context of the sanctions imposed on Iraq from 1990 to 2003 under a UN mandate. Ineffectiveness and humanitarian crises then led to the belief that comprehensive sanctions should be replaced by so-called targeted or smart sanctions, which focus solely on key industries or selected individuals.
The sanctions imposed in connection with the Russian war in Ukraine are currently at the centre of debate. As the Security Council is paralysed by Russia’s veto power, a number of actors have taken this step. Among the key players have been the EU, a number of EU member states implementing EU sanction packages, and other states acting unilaterally, such as the United States. This current example, in the continuity of decades-long sanction practices by states, repeatedly raises questions about the legality of sanctions outside the mandate of the UN Security Council (hereinafter referred to as unilateral sanctions). As will be discussed in more detail in the next section, this paper understands unilateral sanctions not only as sanctions imposed by one state against another but also as sanctions imposed by groups of states such as the EU when such a mandate is lacking.
In matters of military intervention, the Security Council clearly has a monopoly position. However, this is controversial with regard to non-military measures. If a state is injured by the violation of the law by another state, it is in principle undisputed that the latter may enact reciprocal sanctions. In such a case, one speaks of a state’s right to self-help. Often, however, a situation presents itself in which the imposition of sanctions by the injured state itself is not possible, or has little prospect of success. Moreover, there are situations in which no state is itself injured because the victim of the breach of law is the civilian population. The question of whether, or under what conditions, other states (so-called third states) can also react to the violation of the law by means of sanctions is a legal grey area. However, as will be discussed in more detail in the following sections, the overwhelming opinion in the literature is that no general rule exists that prohibits unilateral sanctions per se. Therefore, states may impose sanctions, provided that there is no specific rule of international law to the contrary.
The latter holds true even in light of a significant number of UN General Assembly resolutions condemning unilateral coercive measures. Since the 1990s, unilateral coercive measures have faced, and continue to face, continuous criticism from the General Assembly for their illegality under international law and their negative impact on human rights and the economies of developing countries. Reflecting on this, the question was raised by the then UN Special Rapporteur Idriss Jazairy in his report as to whether this signalled the emergence of a new prohibition under customary international law. While this is widely rejected in legal scholarship, significant political value must be attached to the strong rejection by the UN General Assembly. In contrast, legal value can arise from this rejection only if the conditions for the emergence of customary international law were met: an ongoing and general state practice (read: states would thus have to actually refrain from sanctions), which is based on a state’s legal conviction (read: states would have to express that they refrain from sanctions, because they are unlawful).
Since the emergence of such a general prohibition on sanctions is rejected, their legality must be determined in each concrete individual case in conjunction with a multitude of rules of international law. Only if a sanction in its concrete shape meets the requirements of the relevant norms would it be lawful. It should be noted that the concept of sanctions is very broad, and that its various shapes are subject to very different legal rules. This will be discussed further in Section 1 below. Sections 2 and 3 will then set out in two steps which rules of law are relevant to the various sanctions. The international legality of a sanction can only be determined if it has first been established which obligations the acting state has in the first place, and whether these contradict the specific sanction. There may also be sanctions that do not affect any obligation under international law from any point of view; one example often cited of this is the termination of voluntary aid programmes.
First of all, every state — irrespective of any international treaty obligation — is bound by those rules that apply under customary international law. Customary international law relevant to sanctions is, in particular, the prohibition of intervention, as well as some human rights obligations. Apart from that, the legality of a sanction is determined by the bilateral or multilateral treaties to which a state has committed itself. Therefore, it is quite possible that the sanction imposed by state A on state B is illegal, even though the same sanction would be legal between states B and C. Consequently, a state that is party to few international treaties has more leeway to impose sanctions than one that is bound by many obligations. The term primary obligation will hereinafter be used for this question — i.e. the question of whether a state has an obligation under international law to refrain from taking a measure.
If, after an examination of a state’s primary obligations, it is determined that these contradict the imposition of a sanction, this does not necessarily mean that the sanction is unlawful. Rather, it must be further asked whether there is a justification under international law for this possible violation. Section 3 therefore discusses the question of justifications such as a state’s right to self-help, the right to individual or collective self-defence, and the question of whether groups of states such as the EU have a privileged position compared to individual states.
 Alain Pellet and Alina Miron, “Sanctions”, in Max Planck Encyclopedia of Public International Law (August 2013), marginal no. 26.
 Tom Ruys, “Immunity, Inviolability and Countermeasures – A Closer Look at Non-UN Targeted Sanctions”, in Tom Ruys and Nicolas Angelet (eds.), The Cambridge Handbook of Immunities and International Law (Cambridge University Press, 2019), p. 670.
 See SC Res S/RES/661 (6 August 1990).
 Matthew Happold, “Targeted Sanctions and Human Rights”, in Matthew Happold and Paul Eden (eds.), Economic Sanctions and International Law (Bloomsbury, 2016), p. 88.
 However, the terminology in the literature is not consistent at this point. In some cases, EU sanctions are referred to as multilateral, see e g. Alexander Orakhelashvili, “The Impact of Unilateral EU Economic Sanctions on the UN Collective Security Framework: The Cases of Iran and Syria”, in Ali Z. Marossi and Marisa R. Bassett (eds.), Economic Sanctions under International Law (Springer, 2015).
 Such sanctions are legally classified as countermeasures; for more on this see sections 1 and 2 of this study.
 Iryna Bogdanova, Unilateral Sanctions in International Law and the Enforcement of Human Rights (Brill, 2022), p. 309; Tom Ruys, “Sanctions, Retorsions and Countermeasures: Concepts and International Legal Framework”, in Larissa van den Herik (ed.), Research Handbook on UN Sanctions and International Law (Edward Elgar Publishing, 2016), p. 27; Nema Milaninia, “Jus ad bellum economicum and jus in bello economico: The Limits of Economic Sanctions under the Paradigm of International Humanitarian Law”, in Marossi/Bassett, fn. 5, pp. 96–97.
 Barry Carter, “Economic Sanctions”, in Max Planck Encyclopedia of International Law (April 2011), para. 30; Alexandra Hofer, “The Developed/ Developing Divide on Unilateral Coercive Measures: Legitimate Enforcement or Illegitimate Intervention?” 16 Chinese Journal of International Law 175 (2017), p. 212; Paul de Waart, “Economic Sanctions Infringing Human Rights: Is There a Limit?”, in Marossi/Bassett, fn. 5, pp. 137–38;Daniel Joyner, “International Legal Limits on the Ability of States to Lawfully Impose International Economic/Financial Sanctions”, in Marossi/Bassett, fn. 5, p. 86; cf. Rahmat Mohamad, “Unilateral Sanctions in International Law: A Quest for Legality”, in Marossi/Bassett, fn. 5; for further discussion on this question see Section 220.127.116.11 of this study.
 “Declaration on the Inadmissibility of Intervention in the Domestic Affairs of States”, GA Res 2131 (XX) (21 December 1965); “Declaration on Principles of International Law Concerning Friendly Relations and Cooperation Among States”, GA Res 26/25 (24 October 1970); “Charter on the Economic Rights and Duties of States”, GA Res 3281 (XXIX) (12 December 1974); “Declaration on the Inadmissibility of Intervention and Interference in the Internal Affairs of States”, GA Res 36/103 (9 December 1981); “Economic Measures as a Means of Political and Economic Coercion against Developing Countries”, GA Res 46/210 (20 December 1991), as well as resolutions with the same title from the following years; “Human Rights and Unilateral Coercive Measures”, GA Res 51/103 (12 December 1996), as well as resolutions with the same title from the following years, the most recent of which being GA Res A/RES/76/161 (7 January 2022); GA Res 46/5 (23 March 2021); GA Res 43/15 (22 June 2020); GA Res 40/3 (21 March 2019).
 HRC, “Report of the Special Rapporteur on the negative impact of unilateral coercive measures on the enjoyment of human rights”, Idriss Jazairy, A/HRC/30/45 (10 August 2015), para. 47.
 For more on this, see Section 18.104.22.168 of this study.
 Tullio Treves, “Customary International Law”, in Max Planck Encyclopedia of Public International Law (November 2006), para. 10.
 D. Joyner, fn. 8. p. 86.
 Such measures are referred to as retortions, see International Law Commission, “Draft Articles on Responsibility of States for Internationally Wrongful Acts, with Commentaries 2001”, Yearbook of the International Law Commission, 2001, vol. II (Part Two) (ILC Commentary), p. 128 (introduction to Articles 49–54 of the Articles on Responsibility [ASRIWA]); Ruys, fn. 7, p. 24.
 See Ruys, fn. 7, pp. 24–25.