New Article on the Legality of Signature Strikes

New Article on the Legality of Signature Strikes

The article, which is available in draft form on SSRN, is entitled “‘One Hell of a Killing Machine’: Signature Strikes and International Law.”  It is forthcoming in the Journal of International Criminal Justice as part of a mini-symposium on targeted killing edited by Cornell’s Jens Ohlin.  Here is the abstract:

The vast majority of drone attacks conducted by the U.S. have been signature strikes – those that target “groups of men who bear certain signatures, or defining characteristics associated with terrorist activity, but whose identities aren’t known.” In 2010, for example, Reuters reported that of the 500 “militants” killed by drones between 2008 and 2010, only 8% were the kind “top-tier militant targets” or “mid-to-high-level organizers” whose identities could have been known prior to being killed. Similarly, in 2011, a U.S. official revealed that the U.S. had killed “twice as many ‘wanted terrorists’ in signature strikes than in personality strikes.”

Despite the U.S.’s intense reliance on signature strikes, scholars have paid almost no attention to their legality under international law. This article attempts to fill that lacuna. Section I explains why a signature strike must be justified under either international humanitarian law (IHL) or international human rights law (IHRL) even if the strike was a legitimate act of self-defence under Article 51 of the UN Charter. Section II explores the legality of signature strikes under IHL. It concludes that although some signature strikes clearly comply with the principle of distinction, others either violate that principle as a matter of law or require evidence concerning the target that the U.S. is unlikely to have prior to the attack. Section III then provides a similar analysis for IHRL, concluding that most of the signature strikes permitted by IHL – though certainly not all – would violate IHRL’s insistence that individuals cannot be arbitrarily deprived of their right to life.

I thoroughly enjoyed writing the article, which allowed me to put into academic form a number of ideas I’ve blogged about over the years — the relationship between the jus ad bellum and IHL/IHRL; the definition of armed conflict; what it means to be a member of an organized armed group; the scope of direct participation in hostilities; whether targeting in non-international armed conflict is geographically limited; and the best understanding of imminence under IHRL.  My guess is that both progressives and conservatives will find much to dislike.  Progressive won’t like my conclusion that a number of signature strikes are legal under either IHL or IHLR.  And conservatives won’t like my conclusion that many signature strikes violate both IHL and IHRL, with strikes in the latter category possibly amounting to crimes against humanity.

The deadline to go to press is quite soon, so I’m not sure if I can incorporate reader comments.  But, as always, I would deeply appreciate them.

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Robert Clarke

Interesting reading indeed – but I have a query regarding the matter of criminal responsibility. I agree that the drone operator, for the reasons discussed, is probably a dead end in terms of prosecution. But does that completely dispose of the matter? I would be interested to know what the position is vis-a-vis those who set the program in motion. Are there not arguments to be made there under both the ICC Statute (especially article 25(3)(a)) and customary law?
We know from the High Command Case, which you’ve discussed elsewhere, that there are precedents for holding responsible those concerned in the formulation of orders which are ‘susceptible … of being made criminal in their implementation’. Of course, the intention to kill protected people was much more of a bright line in that case, but it is noteworthy that the evidence against Warlimont in that case included his setting the spurious criteria for German forces to determine when to kill Allied commandoes and when to take them prisoner.

Kevin Jon Heller


It’s an excellent question.  I simply didn’t have time/words to discuss anyone other than the drone operator him- or herself.  It may be easier to prosecute the higher-ups, though I think the mistake issue would help them, too — at least regarding war crimes.

Robert Clarke

Thanks Kevin, possibly a question to be dealt with another day. I’m inclined to agree at least regards war crimes. However, the view emanating from the ICC at the moment seems to be that if one can establish the existence of a state policy to commit the predicate acts constituting crimes against humanity, that will also establish sufficiently close cooperation to engage common responsibility under Article 25(3)(a) and (d), at least as regards the policy makers. Even, it seems, where that cooperation falls short of joint criminal enterprise.


[…] at Opinio Juris, Kevin Jon Heller notes a new draft article he has posted on SSRN regarding the legality of signature strikes under […]


A particularly well articulated analysis of the IHL issues, especially the nuanced discussion of various signatures. Thank you.

On the IHRL analysis, you assert that a single civilian “collateral” death would render an attack unlawful, since proportionality is foreign to IHRL. This seems a strong claim: would a police operation that causes an accidental death in a domestic criminal law enforcement context violate IHRL and be “unlawful” as an arbitrary deprivation of life, or would it simply be unfortunate? I’m not sure that a good faith mistake by police, or a drone operator, that causes a collateral death, necessarily equates to an unlawful act. Thoughts?

Benjamin Davis
Benjamin Davis

Way to go!  The problematic of signature strikes.

Alister Wm Macintyre

I have added your article to my reading list … I have a few other drone articles and national security papers ahead of it.

Lesley Wexler
Lesley Wexler

  You do a wonderful job disaggregating the various types of signatures strikes. It might be helpful to the reader to remind them as to whether the U.S. accepts certain interpretations of International Law or believes itself to be bound by them.  For instance, when you discuss API, it would be nice to remind the reader if the US accepts articles 50-53 as binding customary international law.