Do Attacks on ISIS in Syria Justify the “Unwilling or Unable” Test?

by Kevin Jon Heller

Almost three years ago to the day, I critiqued an article by Ashley Deeks that argued the right of self-defence under Art. 51 of the UN Charter extends to situations in which states are “unwilling or unable” to prevent non-state actors from using their territory to launch armed attacks. As I noted in my post, Deeks herself admitted that she had “found no cases in which states clearly assert that they follow the test out of a sense of legal obligation (i.e., the opinio juris aspect of custom).”

When Deeks wrote her article, ISIS did not yet exist — and the US and other states had not started attacking ISIS in Syria. It is not surprising, therefore, that Deeks is now relying on the international response to ISIS to argue, in the words of a new post at Lawfare, that “the ‘unwilling or unable’ test is starting to seem less controversial and better settled as doctrine.”

There is no question that the US believes the “unwilling or unable” test is consistent with Art. 51. As Jens noted a few months ago, the US officially invoked the test with regard to ISIS and the Khorasan Group in a letter to the Security Council. Moreover, the UK seems to agree with the US: according to Deeks, the UK submitted a similar Art. 51 letter to the Security Council, stating that it supports international efforts to defend Iraq “by striking ISIL sites and military strongholds in Syria” — a position that, in her view,”implicitly adopts the ‘unwilling or unable’ test.”

Deeks does not provide a link to the UK letter, but I have little doubt she is characterizing the UK’s position accurately. I have significant issues, though, with the rest of her post, which argues that three other factors related to the international response to ISIS support the “unwilling or unable” test. Let’s go in order. Here is Deeks’ first argument:

[S]tates such as Jordan, Bahrain, Qatar, and the UAE, which also have undertaken airstrikes in Syria, presumably are relying on the same legal theory as the United States and UK.  (That said, those states have not proffered clear statements about their legal theories.)

There are two problems with this claim. To begin with, even if the four states are relying on “unwilling or unable” to justify their attacks on ISIS in Syria, they have not said so publicly — which means that their actions cannot qualify as opinio juris in support of the test. The publicity requirement is Customary Law 101.

More importantly, though, and pace Deeks, it is actually exceptionally unlikely that these states support the “unwilling or unable” test. All four are members of the Non-Aligned Movement (NAM), which has consistently rejected the test, most notably in response to Turkey’s cross-border attacks on the PKK in Iraq (see Ruys at p. 431):

We strongly condemn the repeated actions of Turkish armed forces violating the territorial integrity of Iraq under the pretext of fighting guerrilla elements hiding inside Iraqi territory. … We also reject the so-called ‘hot pursuit’ measures adopted by Turkey to justify such actions that are abhorrent to international law and to the norms of practice amongst States.

To be sure, Jordan, Bahrain, Qatar, and the UAE have not rejected the “unwilling or unable” test since 9/11 — the statement by NAM above was made in 2000. But there is little reason to believe that their understanding of Art. 51 has fundamentally changed over the past decade. On the contrary, all four are also members of the Arab League, and in 2006 the Arab League formally rejected the “unwilling or unable” test in the context of Israel’s attacks on Hezbollah in Lebanon (see Ruys at p. 453).

Here is Deeks’ second argument:

Iraq vocally has supported strikes within Syria.

This is not surprising, given that ISIS is using Syria as a base for attacks on Iraq. But does Iraq’s support for airstrikes on ISIS in Syria count as opinio juris in favour of the “unwilling or unable” test? I doubt it. After all, not only is Iraq a member of the Arab League, it consistently denounced Turkey’s attacks on PKK bases in its territory between 2007 and 2008 as inconsistent with its sovereignty (see Ruys at p. 461). Iraq’s attitude toward the “unwilling or unable” test thus seems driven exclusively by political opportunism; there is no indication that it considers the test to represent customary international law.

Here is Deeks third argument:

Syria itself has not objected to these intrusions into its territory.

This factor seriously complicates Deeks’ argument. Another word for “not objecting” is “consenting.” And if Syria is consenting to attacks on ISIS in its territory, it is problematic to simply assume — as Deeks does — that all such attacks provide evidence in favour of the “unwilling or unable” test. The US and UK may not want to rely on Syrian consent to justify their attacks. But it seems likely that Jordan, Bahrain, Qatar, and the UAE are relying on Syrian consent rather than Syrian unwillingness or inability to justify their attacks on ISIS in Syria — particularly given their traditional narrow understanding of Art. 51.

Finally, it is important to note what Deeks says immediately after claiming that “[i]In view of these developments, the ‘unwilling or unable’ test is starting to seem less controversial and better settled as doctrine”:

Whether other European states ultimately commit to airstrikes in Syria will be informative; to date, states such as France, Denmark, and Belgium only have provided support to strikes against ISIS within Iraq, not Syria.

This is an important admission, because it means that a member of P-5 and two other important Western states have suggested they are not comfortable with using the “unwilling or unable” test to justify attacks on ISIS in Syria.

So, to recap: the US and UK clearly support the “unwilling or unable” test; Jordan, Bahrain, Qatar, and the UAE are likely basing their willingness to attack ISIS in Syria on Syrian consent; Iraq has a completely opportunistic approach to the “unwilling or unable” test; and France, Denmark, and Belgium seem to reject the test, even if they have not done so explicitly.

And yet we are supposed to believe that the “unwilling or unable” test “is starting to seem less controversial and better settled as doctrine”?

7 Responses

  1. The Australian PM also seems to have some doubts about the legal basis for action against ISIS in Syria:

    Mr Abbott said the fact that the legitimate government of Iraq had invited Australia to assist in defeating an insurgent force, made Australia’s participation in the air and ground war legal.

    “I’m not ruling out some action in Syria but it is not part of the government’s current intentions because, as I’ve said quite frequently over the last few days, the legalities of operations in Syria are quite different from the legalities of operations in Iraq,” he said.

  2. There is no “unwilling or unable” limitation on the inherent right of self-defense, or in this case collective self-defense with Iraq, when armed attacks have begun. As long as the NSA is engaged in ongoing attacks from, partly, Syria, the NSA can be engaged in Syria as well as in Iraq. This has been know prior to the Caroline incident!

  3. Curious one can discuss so many States without even mentioning Isreal. I would advocate a more coherent approach, particularly as unaddressed neighboring issues are partially dispositive.

  4. Thank you for a very interesting post!

    Despite being an interesting read, I do think that you are yourself jumping a bit too much to unsupported conclusions in certain aspects of your analysis. You mention, for example, that Denmark has agreed to operate in Iraq but not in Syria and, consequently, that Denmark along with other countries “seem to reject the test, even if they have not done so explicitly.” Actually, the Danish government has made it explicitly clear that its choice to operate in Iraq and not Syria was a political rather than legal decision. For example, on a question on whether it was concerns over international law that led to this choice, the Prime Minister explained that Denmark was operating where it had been asked to operate, and that “it is not international law that is the reason for us not going into Syria as well.” (my translation)
    So, I do not think that you can so easily take the focus on Iraq by certain countries as a clear indication of them “rejecting” the ‘unable and unwilling’ test. That link needs to be substantiated significantly before you can draw such conclusions. Indeed, it could be argued that Denmark might actually have condoned the ‘unable and unwilling’ test, even if it has not done so explicitly.

  5. Mark,

    You made a good point — until the final sentence. If there is no basis to infer from Denmark’s refusal to participate in attacking ISIS in Syria that it rejects the “unwilling or unable” test, there is even less of a basis to infer from the Prime Minister’s statement that it supports the test.

    And regardless, my argument is vastly more careful than Deeks. I said that the countries “seem” to reject the “unwilling or unable” test; she simply assumes that any state that attacks ISIS in Syria must therefore support it.

  6. Well.. I don’t think I agree with that. There is a limited basis for inferring from Denmark’s lack of participation in Syria a negative statement on the ‘unable and unwilling’ test, given the fact that the Prime Minister explicitly said that the legality of such an operation was not a concern for Denmark. On the contrary, there would seem to be an argument to be made for the fact that an explicit statement saying that the legality of such an operation was not a concern, could indicate that Denmark might support the ‘unable or unwilling’ test, given the fact that there are few obvious alternative legal arguments to be made for the legality of such an operation.

  7. Kevin,

    ‘To begin with, even if the four states are relying on “unwilling or unable” to justify their attacks on ISIS in Syria, they have not said so publicly — which means that their actions cannot qualify as opinio juris in support of the test. The publicity requirement is Customary Law 101.’

    While I am sympathetic to your view that opinio juris should be publicised, I am unaware of strong authority on the point. A quick re-read of the ILA’s report on the formation of CIL this morning ( has left me uncertain on the point.

    While it appears clear that State practice must not be secret, are you aware of any authority to support the view that opinio juris must be public?

    thanks, Ian

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