It’s no secret that several nations (including the United States) are arming or otherwise supporting rebel groups in Syria, and that Russia is doing the same in eastern Ukraine. The international legal debate about these actions has focused on their legality under the jus ad bellum. A separate but related issue has received very little attention: no matter whether a state violates international law by supporting rebel groups abroad, its support might trigger an obligation to try to prevent the groups from committing atrocities.

This obligation—commonly termed an “obligation to protect”—is well established in international human rights law. (See here for a fuller discussion.) The obligation paradigmatically requires a state to try to prevent private actors in its territory from intruding on human rights. The obligation’s precise content is highly context-specific, but for example, a state might have to clamp down on domestic violence or try to incapacitate someone who is about to detonate a bomb. To be clear, the state need not actually prevent this violence. The state is responsible only if it does not take reasonable measures—or, as sometimes said, exercise due diligence—to try to prevent the violence.

Although the obligation to protect typically applies in a state’s own territory, international courts have also applied it extraterritorially—in particular, to states that substantially support rebel groups in other states. (I discuss these cases in more detail in a forthcoming article, especially at pages 270-74.) The most significant decision on the extraterritorial application of the obligation to protect is the judgment of the International Court of Justice in the Genocide Case . The court decided that the former Federal Republic of Yugoslavia (FRY) was not responsible for the genocidal acts at Srebrenica because FRY officials did not exercise sufficient control over the Bosnian Serbs who committed genocide. In other words, the genocide was not itself attributable to the FRY. Nevertheless, the court found that FRY officials should have tried harder to prevent genocide, given their enormous support for the perpetrators. The FRY failed to satisfy its obligation to protect. The ICJ rooted this obligation in the Genocide Convention’s specific text, but similar obligations have been interpreted into other instruments.

Consider two decisions under the European Convention on Human Rights: Ilaşcu v. Moldova arose out of the human rights abuses that Moldovan separatist groups committed in Moldova. The European Court of Human Rights (ECtHR) determined that these groups “survive[d] by virtue of [Russia’s] military, economic, financial and political support,” and that Russia was responsible. In Cyprus v. Turkey, the same court held Turkey responsible as a result of human rights abuses committed by the Turkish Cypriot administration in Turkey.

Both of these decisions are ambiguous on the grounds for finding the relevant states responsible. The ECtHR was unclear on whether Russia and Turkey were responsible because the human rights abuses were attributable to those states or because the states failed to satisfy their obligations to protect. In either event, the states should have done more to prevent the abuses.

The same might be said of the current situations in Iraq, Syria, and Ukraine: even if the supporting states are not directly responsible for the rebel groups’ misconduct, they should try to prevent the groups from violating rights. For example, these states might put in place standards or processes that inhibit the groups that receive the aid from committing atrocities. (Readers might wonder about the obligation’s extraterritorial application where genocide is not at issue. See pages 262-67 of my article for my views.)

How might the supporting states be held responsible? In a number of ways. Human rights courts might get involved, as the ECtHR did in Ilaşcu and Cyprus. Likewise, the bodies that are established under the universal human rights treaties might focus on the issue during their routine report-and-comment processes. And civil society groups might pressure supporting states to impose human rights controls on the rebel groups that receive their aid.