When we left things in the previous post, the Supreme Court had just decided that the Court of Appeals had been in error by examining whether the UN system allowed for sufficient alternative methods for obtaining relief. Rather than applying the ESA cases of Beer and Regan v. Germany (1999) and Waite and Kennedy v. Germany (1999), the lower court should have applied Behrami and Behrami v. France (1999). There is no right to a fair trial when suing the United Nations for peacekeeper action or inaction. Ever.
To get past that problem, the plaintiffs argued - essentially - that this rule does not extend to cases where a violation of ius cogens is alleged. To refute this proposition, the Supreme Court cites two precedents. On the one hand, there's the ECtHR case of Al-Adsani v. UK (2001), and on the other hand there is the recent ICJ ruling in Germany v. Italy (Jurisdictional Immunities of the State). Both courts found, probably correctly, that there is currently no rule of customary international law that prevents states from claiming immunity in cases dealing with violations of ius cogens.
Assuming that this is correct - and I wouldn't dare assume otherwise - why does this govern a case against the United Nations? In par. 4.3.14 the Supreme Court offers nothing more than ipse dixit. I would think, however, that this makes all the difference. After all, the original source of the concept of ius cogens is art. 53 of the Vienna Convention on the Law of Treaties (1969):
A treaty is void if, at the time of its conclusion, it conflicts with a peremptory norm of general international law. For the purposes of the present Convention, a peremptory norm of general international law is a norm accepted and recognized by the international community of States as a whole as a norm from which no derogation is permitted and which can be modified only by a subsequent norm of general international law having the same character.Now this provision does not as such apply to the UN Charter, because the Vienna Convention is not retroactive (cf. art. 4). However, it does strongly suggest that there is a difference between applying a rule of ius cogens to a treaty and applying it to any other kind of legal dispute. In the cases cited above, Kuwait and Germany were invoking a rule of customary international law to avoid liability for an alleged violation of ius cogens. In the Srebrenica litigation, the UN is invoking a provision of a treaty, specifically art. 105 Charter.
If the sequence were reversed, i.e. if the Vienna Convention were older than the UN Charter, the overall applicability of the former to the latter would by fairly headache-inducing, but at least the rule of art. 53 would probably be considered unproblematic. Back in our reality, the law of ius cogens seems to apply quite straightforwardly - through a bit of CIL magic - to pre-1980 treaties. That is certainly true for the ban on genocide, with its venerable Nuremberg pedigree. Even if this is not the case, it would behove the Dutch Supreme Court to explain why not. They shouldn't have simply stopped after discussing ius cogens and state immunity. Those two are very much not the same thing. Just because par in parem non habet jurisdictionem, doesn't mean a state entity like a court can't go after the UN.
Of course, none of that would have ultimately saved the plaintiffs' case. After all, they were not claiming that the UN committed genocide or torture. As I understand it, they're just claiming that the UN were negligent in preventing the genocide. When it comes to any ius cogens exception to art. 105 Charter, the difference between committing genocide and negligently allowing someone else to is big enough to drive a tank through. So in my opinion, the Supreme Court should have held (or referred back to the Court of Appeals so that they could hold) that there is a narrow exception for ius cogens violations, but that no such violation was alleged here.