Ryan Goodman raised a great question yesterday about the US-Russia deal on Syria: may the United States coordinate military operations with Russia if Russia is highly likely to engage in serious violations of the law of armed conflict when conducting its part of the operations? The question might be moot if the deal unravels, which seems to be happening. And because its details are not public, we don’t know exactly how the United States and Russia would coordinate on Syria. Still, it’s worth pondering the legal risks for the United States, as it thinks about its next steps in Syria.

I see two plausible bases for the United States to be responsible under international law as a result of Russian violations. First, the United States might be responsible if it aids or assists Russia in violating the law of armed conflict. Under the customary law on state responsibility, a state is responsible if it: (1) intentionally and/or knowingly, (2) gives another state aid or assistance, (3) that contributes to some internationally wrongful conduct, (4) that the assisting state may not itself commit. In other words, the United States might be responsible if it helps Russia do what it itself may not do—here, engage in gross violations of the law of armed conflict.

As Miles Jackson and Ryan Goodman have explained, there is an ongoing debate about how stringently to apply the first prong of the aiding and assisting test. In particular, it is not clear whether the assisting state is responsible only if its officials intend to facilitate the violations or also if they know that their assistance will contribute to the violations. The commentary that accompanies the authoritative Draft Articles on State Responsibility arguably endorses a knowledge standard. But even if the standard is one of intent, I think Miles and Ryan are absolutely correct: a state’s intent might be inferred if the state knows that its assistance will almost certainly contribute to the recipient’s violations.

The second basis for US responsibility is more of a stretch but still within the realm of possibility. US support for Russian operations might trigger what are sometimes called “obligations to protect.” In general, these obligations require states to take reasonable steps—or, as sometimes stated, exercise due diligence—to prevent third parties from engaging in wrongful conduct. Obligations to protect have a textual hook in the 1949 Geneva Conventions: common Article 1 of the Conventions requires states to “ensure respect” for these conventions “in all circumstances.” The ICRC and many others interpret that language to mean that all states must to try to avert violations of the law of armed conflict, no matter where or by whom the violations are committed. That interpretation is sweeping and, at least for now, seems to be mostly aspirational.

There is, however, a narrower interpretation that has some support in practice and might apply in this context. (See pages 270–74 of this article for a fuller discussion.) A state might have an obligation to protect if it substantially supports the third-party perpetrator. Thus, US support for Russian military operations—for example, the regular provision of targeting intelligence to Russia—might itself generate an obligation to protect. This obligation would fall on the United States, even if it does not also fall on all other states, because the United States would be entangled with and supporting Russia. US responsibility would flow not from Russia’s violations as such but from the United States’ own failure to take adequate steps to try to prevent the violations.

The challenge that the United States would confront in avoiding such responsibility would be in identifying reasonable measures for curtailing Russian violations. Given Russia’s conduct to date and the current state of US-Russia relations, any measures that the United States identifies are likely to be either severely inadequate to protect human life or infeasible. Of course, the United States also might avoid this responsibility by declining to support Russian military operations. But that option presumably is a non-starter so long as the United States wants to preserve the deal.