Stripping foreign fighters of their citizenship: International human rights and humanitarian law considerations could deprive someone of nationality under domestic law,45 but that deprivation cannot affect that person’s humane treatment under international humanitarian law. A clearer link between the measure and international humanitarian law can be identified in the following context: according to Rule 158 of the Customary International Humanitarian Law Study of the ICRC, Vol. 1: Rules, “States must investigate war crimes allegedly committed by their nationals or armed forces, or on their territory, and, if appropriate, prosecute the suspects. They must also investigate other war crimes over which they have jurisdiction and, if appropriate, prosecute the suspects.”46 It is undisputed that foreign fighters, often as members of groups such as ISIS, have committed the most horrible war crimes.47 It can be argued that if states, rather than investigating the war crimes allegedly committed by their nationals, instead rescind responsibility by depriving them of their nationality and make their former nationals the problem of other actors, they violate this customary international humanitarian law obligation. Indeed, as also briefly mentioned at the end of the previous part, deprivation of nationality removes an important jurisdictional link to try possible offenders, the active nationality principle, and thus undermines the fight against impunity.48 By violating this customary international humanitarian law obligation, states also violate the more general obligation of UN Security Council Resolution 2178, the most authoritative resolution on the phenomenon of FTFs, that “Member States must ensure that any measures taken to counter terrorism [such as deprivation of nationality] comply with all their obligations under international law, in particular international human rights law, international refugee law, and international humanitarian law”.49 45 Although the conclusion of the previous part should be repeated here again, namely that under international human rights law, the measure is highly problematic. 46 J.-M. Henckaerts and L. Doswald-Beck, Customary International Humanitarian Law, Vol. 1: Rules, Cambridge University Press: Cambridge, 2005 (reprinted with corrections 2009), p. 607. 47 See e.g. Eurojust, Network for Investigation and Prosecution of Genocide, Crimes against Humanity and War Crimes, Cumulative Prosecution of Foreign Terrorist Fighters for Core International Crimes and Terrorism-related Offences, The Hague, May 2020, available at: https://www.eurojust.europa.eu/sites/ default/files/Partners/Genocide/2020-05_Report-on-cumulative-prosecution-of-FTFs_EN.PDF, pp. 12–13: “The Reports of the Independent International Commission of Inquiry on the Syrian Arab Republic (UN CoI Syria) stated that ISIS had endangered and directed acts of violence against the civilian population in areas controlled by them, and persons not taking part in hostilities. Accordingly, they committed the war crimes of murder, execution without due process, mutilation, enforced disappearance, torture, cruel treatment, hostage-taking, rape and sexual violence, forced pregnancy, the use and recruitment of children in hostilities and attacking protected objects, forcibly displacing civilians, outrages upon personal dignity as well as other serious violations of IHL [international humanitarian law]. ISIS violated its obligations towards civilians and persons hors de combat which amounted to war crimes by beheading, shooting and stoning men, women, children and captured soldiers. They mutilated their bodies and carried out amputations and lashings in public spaces. Prisoners of ISIS had to survive beatings, whipping, electrocution, and suspension from walls or ceilings. Moreover, ISIS was engaged in abductions, and women and girls suffered from sexual slavery, gang raping, executions for unapproved contact with the opposite sex and stoning for adultery. According to the UN CoI Syria, the commanders of ISIS had wilfully perpetrated these war crimes with the indisputable intent of attacking persons while they were aware of their status as civilians or persons no longer participating in hostilities. Yazidis were especially targeted by horrific abuse by ISIS due to their community’s religious identity [original footnotes omitted].” 48 H. Cuyckens, above note 8, “Domestic prosecution in the country of origin” section. 49 UN Security Council, above note 4, p. 1. 615