Policy Brief - Quarterly Journal: International Security

States Will Not Give Nuclear Weapons to Terrorists

  • Keir A. Lieber
  • Daryl Press
| September 2013

This policy brief is based on "Why States Won't Give Nuclear Weapons to Terrorists," which appears in the Summer 2013 issue of International Security.

Bottom Lines

  • States Won't Give Nuclear Weapons to Terrorists. Giving a nuclear weapon to terrorists would be as suicidal as launching a nuclear strike directly. Evidence shows that guilty terror groups in normal high-casualty attacks are almost always identified. Attribution of nuclear terrorism would be even easier; tracing a guilty group to its state sponsor would be simple. The fear of nuclear attack by proxy is unfounded.
  • Nuclear Terrorism Would Not Remain Anonymous. Seventy-three percent of past high-casualty terrorist attacks have been traced back to the perpetrators. More important, 97 percent of moderate- and high-fatality attacks conducted on U.S. territory or that of a major U.S. ally were attributed. Furthermore, the suspect list of state sponsors would be short: few countries sponsor terrorism; and of the few that do, only Pakistan has nuclear weapons or enough fissile material to manufacture one.
  • Fear of Nuclear Handoff Does Not Justify Costly Military Action. The fear that Iran will transfer nuclear weapons or materials to terrorists does not warrant costly steps like military action to prevent proliferation. To be persuasive, arguments for military strikes need to be based on other potential negative consequences of proliferation.
  • Stop Understating U.S. Nuclear Attribution Capabilities. The United States has gaps in its "nuclear forensics" capabilities, which should be addressed. The strength of U.S. attribution capabilities, however, lies elsewhere—in the day-to-day intelligence gathering that routinely thwarts other terror strikes. History shows that U.S. attribution capabilities are very strong, and saying otherwise undermines deterrence.

States Won't Give Nuclear Weapons to Terrorists

Nuclear terrorism is often described as the single biggest threat to U.S. national security. The fear is 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.

Assessing the risk of nuclear attack-by-proxy turns on the question of whether a state could sponsor nuclear terrorism and remain anonymous. A leader could rationalize such an attack—and entrust terrorists with a vitally important mission—only if doing so allowed the sponsor to avoid retaliation. After all, if a leader did not care about retaliation, he or she would likely conduct a nuclear strike directly. Giving nuclear weapons to terrorists makes sense only if there is a high likelihood of remaining anonymous after the attack.

Fortunately, neither a terror group, nor a state sponsor, would remain anonymous after a nuclear terror attack. 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.

Nuclear Terrorism Would Not Remain Anonymous

Seventy-three percent of high-casualty terrorist attacks have been traced back to the perpetrators, and almost all—97 percent—of the moderate- or high-fatality attacks on U.S. or allied territory are attributed to the guilty organization. Evidence from history reveals a strong link between the number of fatalities and the likelihood of attribution; the greater the consequence of an attack, the more resources countries have devoted to identify and find those responsible. A nuclear terror attack would trigger an unprecedented search employing every diplomatic, intelligence, and military tool available. Furthermore, victims of terrorism and their allies would have little difficulty tracing culpability from the guilty terror group to the state sponsor.

Countries contemplating giving nuclear weapons to terrorists would face powerful incentives to collaborate with a group with an extensive record of professionalism, a demonstrated ability to conduct complex international operations, and a long history of cooperation and mutual trust. It is implausible that Pakistan would entrust its future to Hezbollah—an Iran-sponsored group. Nor would Iran entrust its survival to Lashkar-e-Taiba—Pakistan-based terrorists. Yet the need to collaborate on such a life-and-death matter with an established, trusted partner greatly simplifies attribution.

Finally, it would not be difficult for the U.S. government to find the culprits after a nuclear incident because investigators would be starting with a very short suspect list: most terror groups have only one or two sponsors; and of the few countries that sponsor terrorism, only Pakistan has nuclear weapons or enough fissile material to manufacture one.

In short, neither a terror group nor a sponsor would remain anonymous after a nuclear terror attack. Attribution would be far easier than analysts and policymakers frequently suggest, and therefore passing weapons to terrorists does not offer countries a loophole to avoid the constraints of deterrence.

Fear of Nuclear Handoff Does Not Justify Costly Military Action

Nuclear proliferation is undesirable for a range of reasons, and hence preventing the spread of nuclear weapons should remain a major U.S. foreign policy goal. But the risk that a country might give nuclear weapons to terrorists is very small, and this fear does not justify costly measures to prevent proliferation. In particular, Iranian leaders would have to be crazy or suicidal to think that they could give a nuclear weapon to one of their terrorist collaborators and avoid catastrophic repercussions. If Iranian leaders were that irrational, the bigger problem would be the possibility of direct nuclear attack without concern for the retaliatory consequences, not the alleged problem of a nuclear handoff.

Stop Understating U.S. Nuclear Attribution Capabilities

U.S. officials and analysts should stop understating the ability of the United States to attribute nuclear terrorist attacks. Many U.S. officials have publicly lamented the limits of U.S. "nuclear forensic" capabilities—that is, the use of a bomb's isotopic fingerprints to trace the fissile material device back to the reactors, enrichment facilities, or uranium mines from which it was derived. They warn that existing nuclear forensic capabilities do not permit the United States to reliably attribute nuclear detonations, and they urge greater spending on those programs. Those programs have merit and perhaps require additional funding, but U.S. attribution capabilities do not rest upon nuclear forensics. Instead, attribution is the result of day-to-day, multisource intelligence, diplomacy, and bare-knuckled coercion. Historically, terrorist incidents are attributed because of cellphone intercepts, computer surveillance, human intelligence, routine investigative work, and tips from allies. Those mundane tools are the basis for the 97 percent attribution rate, which applies to even moderately destructive attacks on U.S. and allied territory. The investigative effort after a nuclear incident would be unprecedented.

Disparaging assessments of U.S. attribution capabilities are not merely inaccurate; they inadvertently undermine deterrence by misleading enemies to overestimate the feasibility of an anonymous attack. The best way to deter countries from passing weapons to terrorists is to demonstrate the ease of nuclear attribution and the devastating consequences that would befall the sponsoring state.


Keir A. Lieber is Associate Professor in the Edmund A. Walsh School of Foreign Service and the Department of Government at Georgetown University.


Daryl G. Press is Associate Professor of Government at Dartmouth College.


International Security is America’s leading peer-reviewed journal of security affairs. It provides sophisticated analyses of contemporary, theoretical, and historical security issues.

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Statements and views expressed in this policy brief are solely those of the authors and do not imply endorsement by Harvard University, the Harvard Kennedy School, or the Belfer Center for Science and International Affairs.

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For Academic Citation: Lieber, Keir A. and Daryl G. Press. “States Will Not Give Nuclear Weapons to Terrorists.” Policy Brief, Quarterly Journal: International Security, September 2013.

The Authors