Professor Ashley Deeks on the Law of Pre-emptive Strikes

One of the most contested questions in international law is whether and when it is lawful for a state to use force before it suffers an armed attack. University of Virginia School of Law professor Ashley Deeks recently penned a chapter on the debate for "The Oxford Handbook of the Use of Force in International Law," which is slated for release in January.

Deeks is an expert in international law who previously served as the assistant legal adviser for political-military affairs in the U.S. Department of State's Office of the Legal Adviser, where she worked on issues related to the law of armed conflict, the use of force, conventional weapons, and the legal framework for the conflict with al-Qaida.

The debate over pre-emptive strikes took on particular salience in 2002, Deeks said, when the United States claimed — more clearly and assertively than before — that a state could use force to forestall certain hostile acts by its adversaries.  More than a decade later, Deeks' chapter, "Taming the Doctrine of Preemption," reviews where the debate currently stands and where it is heading.

How have the legal grounds for pre-emptive strikes by one state against another state or a non-state actor such as al-Qaida evolved in the years since the Sept. 11 attacks?

One of the longest-running debates in international law is when a state may use force against another state or a non-state armed group beforeit has been attacked. The U.N. Charter makes clear that a state may respond forcibly after it suffers an armed attack; that presents a clear case of self-defense. But what happens if a state learns that it is about to become the victim of an armed attack? Must it wait to suffer the blow before it responds? Most states and scholars agree that the answer is no: States can respond in what's often called "anticipatory self-defense." The traditional test about when anticipatory self-defense is appropriate flows from an 1842 dispute between the United States and United Kingdom known as the Caroline case. Then-Secretary of State Daniel Webster asserted that using force before an armed attack actually had transpired could only be justified in cases in which the "necessity of that self-defense is instant, overwhelming, and leaving no choice of means, and no moment for deliberation." Many summarize this as a requirement that the attack be "imminent."

That test worked reasonably well when we were talking about state-on-state violence, where it is possible to observe troop movements, activity at missile silos and launch pads, and deployments of military equipment. The Caroline test poses challenges, though, when we are talking about new technologies (such as cyber weapons, which can be launched virtually without warning) and new actors (such as armed terrorist groups, which operate in much smaller groups and under significant secrecy). These challenges have prompted some states — and the United States in particular — to re-consider how to apply the "imminence" requirement. How close in time must the attack be before a state can act in anticipation? And how certain must a state be about the pending attack? These questions are important because we don't want to allow states to use force pretextually, or to use force in cases in which the suspected armed attack is nebulous, or unlikely, or could be deterred in other ways. But states also will not accept a test that forces them to wait so long to act that their response will ultimately prove ineffective.

As a matter of terminology, many people refer to "anticipatory self-defense" as the use of force in self-defense to halt an imminent threat of an armed attack in the sense that Daniel Webster meant it. In contrast, "pre-emptive" self-defense often refers to the use of force in self-defense to halt a particular, tangible course of action that the potential victim state perceives shortly will evolve into an armed attack against it but cannot be said to leave "no time for deliberation." The latter is more distant in time than the former, and therefore is likely to be more uncertain and more inchoate.

How does the relative certainty of an attack affect a nation's legal grounds to conduct pre-emptive self-defense under international law?

One advantage to a pretty strict "imminence" requirement is that it offers a relatively bright line about when anticipatory force is legally acceptable. We should worry about stripping away that bright line if we don't substitute it with another test to cabin state discretion. One way we might cabin that discretion is to establish a set of factors that states should use to assess the propriety of "pre-emptive" self-defense. Scholars have suggested factors such as the severity of the threat; the probability that the threat will materialize; the capacity and intent of the likely attacker; the quality of the future victim state's intelligence about the threat; and whether the future victim first has sought U.N. Security Council support for its action. So the relative certainty of a future attack would be an important element that a state would invoke to justify a pre-emptive forcible action — but that could not and should not be the only consideration.

Does the legal right to use a pre-emptive approach differ based on the nature of the threat?

At this point, international law is definitely in flux about the propriety of pre-emptive self-defense. When the Bush administration in its 2002 National Security Strategy defended the view that the United States could use force "before attacks occur, even if uncertainty remains as to the time and place of the enemy's attack," it was roundly criticized for undermining international law. But some states — including Australia, Japan, Russia and the United Kingdom — since have accepted some version of this theory, without being precise about how they would apply it. And in 2011, then-senior White House official John Brennan asserted that other states are adopting a more flexible understanding of "imminence" when dealing with terrorist groups, because the threats posed by non-state actors do not present themselves in traditional ways.

So the nature of the threat is driving the conversation here. The ability of non-state actors to inflict levels of violence hitherto only achievable by states; the possible proliferation of WMDs to rogue states and terrorist groups; and the unique nature of cyber operations all are affecting the demand for a new approach to pre-attack self-defense. But it is premature to say that states have established a clear and well-defined legal right to use force against developing terrorist threats that have not crystallized into credible and tangible plots. International law develops as a matter of claim and counter-claim, and we will need to see and analyze more practice in this area before we can say with confidence that this is where the law is headed.

Do questions related to pre-emptive or anticipatory self-defense arise in the context of the U.S. campaign against the Islamic State in Syria?

They do. The U.S. legal theory for using force against ISIS in Syria is based on the idea that the United States is acting in "collective self-defense" of Iraq. That is, Iraq has suffered armed attacks from a non-state actor that is partly based in a neighboring state. Those attacks triggered Iraq's right to self-defense against ISIS in Syria, because Syria proved unwilling or unable to suppress the threat posed by ISIS. Iraq asked the United States to support it in its use of self-defense. That theory does not implicate pre-emptive or anticipatory self-defense.

But the U.S. airstrikes against the Khorasan Group in Syria may implicate pre-emptive self-defense. When the United States initiated those strikes in late September, the Pentagon asserted that Khorasan was "nearing the execution stage of launching an attack on Europe or the homeland." Khorasan has not completed any actual attacks on the United States or on U.S. interests; rather, the Pentagon was suggesting that the group was getting close to doing so. Using force against Khorasan therefore would be an action taken in anticipatory or pre-emptive self-defense. To be lawful, those airstrikes would have to be both necessary to defeat the threat and proportionate to the threat that the group poses, and the threat would need to be actual and serious.

On the other hand, if the Khorasan Group is simply a sub-group or off-shoot of al Qaida, the United States presumably would argue that it may target Khorasan members as part of its ongoing armed conflict with al-Qaida. That theory does not require Khorasan members to be plotting new attacks against the United States, but instead treats them as part of an organized armed group with which the United States has been in conflict since at least 2001.

Whichever the U.S. legal theory, it is notable that no state to date has criticized the United States for targeting members of Khorasan. This practice, like other examples of states using force before actually suffering an armed attack, will be relevant to future assessments of the legality and legitimacy of pre-emptive self-defense.