Establishing “Retrospective” State Responsibility for the Crime Against Humanity of Transatlantic Slavery

[Dylan Jesse Andrian is an LL.M. candidate at Harvard University, holding undergraduate law degrees from Universitas Gadjah Mada and Maastricht University. He has worked at the ITLOS Legal Office, the Al Hassan Defence team before the ICC, and has drafted legal opinions for Amnesty International Indonesia, the Indonesian National Human Rights Commission, and the Government of Indonesia in the ICJ Climate Change proceedings.]

On 25 March 2026, the UN General Assembly passed Resolution A/80/L.48, which recognized the transatlantic slave trade as “the gravest crime against humanity.” While this recognition was a great accomplishment in its own right, questions of how to establish responsibility and secure reparations remain on the table. Among the 3 States who voted against and the 52 States who abstained were the States arguably most responsible for the transatlantic slave trade, such as the United States, the United Kingdom, and several members of the European Union. Given that a political road to reparations seems unlikely, this article aims to explore the possibility of obtaining reparations through judicial means.

The principal obstacle is Article 13 ARSIWA, which stipulates States can only be held responsible for acts that contravene international obligations in force at the time the act was committed. Accordingly, a Respondent State may argue against responsibility for the transatlantic slave trade on the grounds that it was not recognized as a crime against humanity at the time of its commission during the late 15^th to early 19^th Century. In fact, three cases relied upon by the ILC to draft Article 13 ARSIWA dealt with the exact issue of slavery’s legality, with the 1854 Enterprize decision finding slavery, though “odious and contrary to the principles of justice and humanity”, nevertheless permissible under international law at the time. (It was only a year later, with the 1855 Lawrence decision that slavery was found contrary to the law of nations.)

So how would litigating the transatlantic slave trade as a crime against humanity get past Article 13 ARSIWA?

Route 1: The Prohibition on Crimes Against Humanity has Existed Since “Time Immemorial”

In 1945, the Nuremberg Tribunals faced a problem of legality. According to the principle nullum crimen sine lege, a person cannot be prosecuted under a law not in force at the time of their supposedly criminal acts. Defendants before the Tribunals argued the crimes for which they were prosecuted only came into existence with the adoption of the Nuremberg Charter in 1945, while their allegedly criminal conduct took place long before then, occurring largely in the late 1930s to the early 1940s.  

The Tribunals (largely) responded by stating that the Nuremberg Charter did not create new law, but merely acknowledged pre-existing law. In Justice and Einsatzgruppen specifically, the Tribunal explained that the Nuremberg Charter was not “original substantive legislation” but rather “procedural means” for prosecuting pre-existing international law “valid long prior to World War II.” 

But exactly how long prior?

In 1962, the Israeli Supreme Court was faced with a similar legality objection in respect of Eichmann’s involvement in Nazi crimes against humanity inter alia: Eichmann argued he could not be held responsible for criminal conduct committed between 1939 and 1945 under an Israeli law passed years later in 1950. The Israeli Supreme Court roundly rejected this argument, holding that the crimes of which he was convicted “must be seen as having constituted since ‘time immemorial,’ a part of international law.” (Fun fact: Chapter 39 of the Statute of Westminster 1275 sets the end of “time immemorial” at 6 July 1189, well before the beginning of the transatlantic slave trade.) Rulings by national courts have affirmed the Nuremberg Tribunals’ conclusions, without disputing Eichmann’s “time immemorial” assertion (see the following U.S. judgments: 1992 Siderman de Blake, 1994 Princz, and 2007 Khulumani).

The drafters of Resolution A/80/L.48 may have had this route in mind when they invoked past anti-slavery instruments, such as the 1235 Kouroukan Fouga (Manden Charter), the 1815 Vienna Declaration on the Abolition of the Slave Trade, and “the more than six-century Africana abolitionist tradition, early acts of resistance and testimony, State diplomacy, armed struggles and the strategic use of courts and petitions to assert human rights, dignity, autonomy over their bodies and territorial sovereignty.” These instruments and practices, while certainly contributing to the development of the law of humanity, could be thought of as simultaneously reflecting the prior existence of that law. 

Following this logic, Article 13 ARSIWA would pose no problem: the transatlantic slave trade was illegal long before it began and the arbitral decisions that made any suggestion to the contrary were incorrect. 

Route 2: The Prohibition on Crimes Against Humanity is not Subject to Temporal Responsibility Restrictions

Strict positivists may insist conduct prior to 1945 cannot be considered crimes against humanity, on the premise that the prohibition of such conduct only crystallized in 1945 with the adoption of the Nuremberg Charter. This point is not totally without merit. For example, in his 1946 Oral Report to the UNGA, the Secretary-General urged that the Nuremberg principles be “made a permanent part of the body of international law as quickly as possible,” implying they were not yet so as late as 1946. Moreover, Hersch Lauterpacht, with whom the notion of crimes against humanity is often credited, himself acknowledged this category of crimes to be “clearly an innovation”.

And yet, the Charter, including its “innovative” crimes provisions, was still applied retroactively. When pressed on this issue in respect of the crime of aggression, the judges of the International Military Tribunal opined that the principle of legality could be set aside to punish a perpetrator who knew what they were doing was wrong. The Tribunal then looked to existing treaties and assurances given of non-aggressive behavior, not as proof of a rule criminalizing aggression, but as proof of the defendant’s knowledge that their conduct was wrong. (Incidentally, nothing suggests notice of moral objectionability can only be deduced from instruments of law; while such instruments are necessarily highly probative, knowledge can and should be assessed with reference to the entire world of available evidence.) 

This is not to assert the principle of legality is automatically displaceable in every case involving international crimes. For example, Articles 22 and 24 of the Rome Statute expressly prohibits prosecution of conduct occurring before the Rome Statute’s entry into force; mere reference to the practice of the Nuremberg Tribunals would not be sufficient to overcome rules of such statutory strength. But Articles 22 and 24 (and similar rules, such as Article 7 ECHR) are specific forum or conventional rules, not rules of general international law. In fact, other arenas, both national and international, seem to accept that justice is upheld where setting aside the legality principle would result in successful prosecution of international crimes. For example, the Committee Against Torture, wrote in its 2009 Concluding Observations regarding Spain: “bearing in mind the long-established jus cogens prohibition of torture, the prosecution of acts of torture should not be constrained by the principle of legality.” Similarly, Article 1 of the 1968 Convention on the Non-Applicability of Statutory Limitations to War Crimes and Crimes Against Humanity proscribes statutory limitations on crimes against humanity “irrespective of the date of their commission” and “even if such acts do not constitute a violation of the domestic law of the country in which they were committed.”

But what bearing does this extended discussion on the principle of legality have on Article 13 ARSIWA? In the first place, it is necessary to recognize that Article 13 did not develop as a freestanding rule of international law. Rather, as the ILC explains in its commentaries, it reflects a “general principle of intertemporal law.” In other words, no one can point to a moment in the lifespan of international law when the logic of Article 13 was inapplicable, followed by a later practice establishing it. On the contrary, the principle emerged from the broader domestic principle (tempus regit actum), common to various national legal systems, requiring all laws, criminal or otherwise, to be published in advance of enforcement. Hans Kelsen, writing on the ex post facto laws applied at Nuremberg, identified this broader rule as the progenitor of the legality principle as well, citing William Blackstone as authority. It stands to reason that if the legality principle, being the stricter of the twin fruits of the tempus regit actum tree, can be relaxed in respect of jus cogens norms, Article 13 should surely admit of a similar flexibility. (For those preferring a path not involving the genealogy of Article 13 ARSIWA, a possible alternative route is to invoke the desirability of systematic integration across international law as a reason to use one rule to understand the other.)

As a counter-point, the ILC commentaries may be cited to argue that the non-retrospective assumption of responsibility applies even to violations of jus cogens norms. However, the ILC’s basis for this claim is Articles 64 and 71(2)(b) of the Vienna Convention on the Law of Treaties, which say nothing whatsoever about non-retrospective assumption of responsibility; rather, they clarify that a treaty in conflict with a peremptory norm is void at the moment the peremptory norm emerges, but is not void ab initio. At most, this may mean certain behavior, such as slavery, may be legal according to the international instruments authorizing it at the time, but consistency of behavior with a single instrument does not necessarily entail legality of that behavior under the totality of general international law. For example, if States conclude Treaty A among themselves to suppress freedom of expression, their subsequent suppressions of that freedom would not violate Treaty A per se; however, such suppressions would fall afoul of customary international law and any human rights treaties to which those States are party. Just the same, consistency of slavery with a handful of international instruments does not automatically mean it is consistent with the other strands of law that make up the tapestry of the international legal order.

Assuming then that the knowledge-of-wrongdoing exception overcomes Article 13 ARSIWA as adeptly as it overcomes the legality principle, the historical anti-slavery instruments cited in the Resolution take on a different shade of purpose: if they cannot reflect the existence of a rule prohibiting slavery at the time of its commission, they can at least prove the perpetrators’ knowledge of slavery’s moral objectionability. One such instrument, the 1815 Vienna Declaration, quite correctly described the slave trade as “repugnant to the principles of humanity and universal morality.”

In any event, no one can seriously argue that a person, stripping another of their freedom and dignity, trafficking them across seas and oceans, and subjecting them to exploitative and deeply inhumane working conditions could be ignorant of the evil they were committing. To repeat the oft-quoted fiction that “no one knew slavery was wrong” is to turn a blind eye to the eloquent fact that the slaves themselves did.