International Relations

Continuing our new series of collaborations with political science journals, we are pleased to present the following guest post from Georgetown University political scientist Keir A. Lieber and Dartmouth College political scientist Daryl G. Press to discuss their article “Why States Won’t Give Nuclear Weapons to Terrorists” that appears in the current issue of International Security. In conjunction with this post, MIT Press will make the article freely available to all for the next 30 days; you can download it here.

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Nuclear terrorism is often described as the single biggest threat to U.S. national security. Analysts and policymakers worry that a hostile state could surreptitiously transfer a nuclear weapon or fissile material to a like-minded terror group, thus orchestrating a devastating attack on the United States or its allies while remaining anonymous and avoiding retaliation. This fear served as a key justification for the invasion of Iraq in 2003, and it helps drive current arguments in favor of a military strike against Iran’s nuclear program.

We assessed the risk of nuclear attack-by-proxy by exploring the likelihood that a state could sponsor nuclear terrorism and remain anonymous. The question of attribution is crucial because a leader could only rationalize such an attack – and the decision to entrust terrorists with a vitally important mission – if doing so allowed the sponsor to plausibly avoid retaliation. If a leader did not care about retaliation, he would likely just conduct a nuclear strike directly – using missiles or special forces for targets beyond missile range. Giving nuclear weapons to terrorist only makes sense if there is a high likelihood of remaining anonymous after the attack.

We undertook two approaches to assess the likelihood of attributing a nuclear terror attack. First, because there is no data on the aftermath of nuclear terrorist attacks, we use the ample data on conventional terrorism to explore post-attack attribution rates – focusing on high-casualty terror attacks. The data, as shown in Figures 1 and 2, reveal a strong positive relationship between the number of fatalities caused in a terror attack and the likelihood of attribution. Roughly three-quarters of the attacks that kill 100 people or more are traced back to the perpetrators. Moreover, attribution rates are far higher for attacks on the territory of the United States or a major U.S. ally – 97% (36 of 37) for incidents that killed 10 or more people.

Second, we explore the challenge of tracing culpability from the guilty terror group back to its state sponsor. Leaders considering giving nukes to terrorists would have a strong incentive to select a group with whom they have a long, trusting relationship – in this case, members of the terror group would need to keep the source of the nuclear weapon a secret indefinitely – and one with a track record of successful operations and professionalism. But those two constraints greatly facilitate the task of tracing an attack from a guilty group to its sponsor. As Table 1 shows, there are few established terrorist groups who have state sponsors; each of them has few sponsors (typically one); and only one country that sponsors terrorism has nuclear weapons or enough fissile material to manufacture a weapon.

Our overarching conclusion is that neither a terror group, nor a state sponsor, would remain anonymous after a nuclear terror attack. Attributing nuclear terror incidents would be far easier than is typically suggested; passing weapons to terrorists, therefore, does not offer countries an escape from the constraints of deterrence. If leaders understand these facts, they will be as reluctant to give weapons to terrorists as they are to use them directly; both actions would invite devastating retaliation.