The Trump administration’s 2018 Nuclear Posture Review adopted perhaps the most aggressive stance to date on the ramifications of state support for nuclear terrorism: “The United States will hold fully accountable any state, terrorist group, or other non-state actor that supports or enables terrorist efforts to obtain or employ nuclear devices.” Although the document acknowledged the limited role of U.S. nuclear weapons in countering nuclear terrorism, it pointedly noted that “adversaries must understand that a terrorist nuclear attack against the United States or its allies and partners would qualify as an ‘extreme circumstance’ under which the United States could consider the ultimate form of retaliation” (emphasis added). Little ambiguity should exist about the implications of this warning.

Central to the efficacy of such deterrent threats is credibility. In the context of traditional nuclear deterrence, credibility requires both the operational ability to conduct a nuclear attack and the political resolve to do so. For the credibility of nuclear forensics, there is an additional requirement—the ability to determine confidently the origin and pathway of illicitly acquired or deliberately provided nuclear materials and to distinguish between these two modes of acquisition. Following an act of nuclear terrorism, the confidence that U.S. forensic capabilities supply in answering these questions would both strengthen the resolve of American leaders to retaliate and help persuade allied and non-aligned governments of the legitimacy of the United States’ reprisal. Hostile states, foreseeing the linearity from technical forensic analysis to U.S. retaliation, may thus be deterred from facilitating nuclear terrorism in the first place.

Calls to develop and refine nuclear forensic capabilities have been enshrined in a succession of U.S. and international policies and legislation for more than three decades. The 2006 National Strategy for Combating Terrorism, for example, included forensics among a list of six objectives to address the threat of weapons of mass destruction (WMD) terrorism. The strategy stated the intention to “develop the capability to assign responsibility for the intended or actual use of WMD via accurate attribution—the rapid fusion of technical forensic data with intelligence and law enforcement information.” More than a decade later, the 2018 National Strategy for Countering WMD Terrorism echoed this requirement but went further, drawing a direct link between forensics and deterrence. “Deterring hostile states and individuals from supporting WMD terrorists requires the means to quickly and accurately attribute such support,” noted the strategy, which pledged to “refine the accuracy, timeliness, and confidence of forensic capabilities to identify the source of … nuclear materials, weapons, or components used in terrorist attacks.”

In the immediate aftermath of a terrorist nuclear attack, little imagination is required to envision the urgency with which policymakers would demand answers as to the origin of the material used and whether a hostile state had provided it wittingly to terrorists. In this context, the importance of speed cannot be overstated; analytic processes that may require weeks to provide confident results would far exceed the window of time in which national leaders would desire—and be expected—to respond. Among the most pressing questions U.S. leaders would face is whether the attack was a one-off event or if subsequent detonations might occur. The rapid ability to identify the source of the nuclear material used would allow the United States to press the source government—now identified as the likely point or origin, if not the culprit, and thus under the threat of retaliation—to answer whether additional material had been supplied or was unaccounted for.

To be sure, nuclear forensic analysis would represent just one element of a post-attack investigation, along with vitally important inputs from the intelligence and law enforcement communities, as well as contributions from allies and international entities such as the International Atomic Energy Agency (IAEA). Yet forensic analysis at the DoE and NNSA national laboratories would yield scientific insights available from no other source. As these capabilities continue to mature, including improvements in the speed with which outputs can be delivered to national decisionmakers, they may be harnessed to resolve uncertainty about a broader range of nuclear incidents, from overtly hostile acts to accidents and other unexplained phenomena.

Forensics in Resolving Hostile Unattributed Nuclear Events. Since the advent of the nuclear age, the threat posed by nuclear weapons has overwhelmingly come from traditional delivery vehicles—first aircraft and later ballistic and cruise missiles. Yet, for almost as long as nuclear weapons have existed, the possibility of clandestine delivery means has also been entertained. Indeed, a 1947 fbi memo concluded that “a complete atom bomb could be smuggled into the United States as freight even though it is of large bulk, and the bomb could be detonated by remote control after it has been placed at a specific location.” Subsequent analyses considered the delivery of nuclear devices via disguised aircraft, small watercraft, and even disassembled weapons smuggled piecemeal within Soviet diplomatic pouches. Although the likelihood is vanishingly small that a state would conduct a clandestine, unacknowledged nuclear attack, more recent developments suggest that forensics programs could be oriented to account for this possibility.

The past several decades have witnessed multiple states’ use of proxy forces to commit acts of aggression, perhaps most infamously Russia’s use of unidentified “little green men” to invade Ukraine and annex the Crimean Peninsula. In other cases, pariah states have supplied non-state groups with increasingly sophisticated weaponry, such as Iran’s provision of advanced arms to the Lebanese militant group Hezbollah and its suspected delivery of drones, ballistic missiles, and surface-to-air missiles to Houthi rebels in Yemen. Although most analysts have concluded that nuclear-armed states would be extremely unlikely to provide nuclear capabilities to proxies, one cannot exclude this possibility altogether. Nor can the scenario be dismissed in which a hostile state’s own personnel conduct a “false flag” operation to give a nuclear detonation the appearance of a terrorist attack. The knowledge that the United States could confidently attribute the responsible government in this scenario may help to discourage consideration of such an attack.

Other potential uses of nuclear weapons in which a state might seek to deny responsibility include single- or limited-use scenarios in a regional conflict. Although little ambiguity might exist over the origin of the attack, a state’s denial of responsibility, however brazen and unconvincing, may complicate the international response. Consider the 2010 sinking of the Republic of Korea Navy corvette Cheonan in the Yellow Sea, which killed forty-six South Korean seamen. Despite significant evidence of North Korea’s guilt and the fact that no plausible party but North Korea could have been responsible, the Kim regime steadfastly denied having struck the vessel. While authorship of a nuclear attack would be considerably more difficult to disavow, scenarios are conceivable in which plausible deniability would provide just enough justification for wavering states to avoid further escalation.

Over the past two decades, the strategic literature has increasingly speculated that Russia may use low-yield nuclear weapons in a conflict as part of its purported “escalate to deescalate” doctrine. In this circumstance, one can imagine the Kremlin nonetheless claiming that a low-yield nuclear strike was in fact conventional, denying responsibility entirely, or perhaps instead blaming the United States or the United Kingdom. Profession of belief or disbelief in such a claim by Russia would break along predictable lines, potentially preventing a broad international coalition from forming to condemn the attack. Notably, the premise of Article 5 of the North Atlantic Treaty Organization (NATO) Charter—that an attack against one NATO member constitutes an attack on them all, requiring a collective response—presupposes that the agent of the attack would be known and agreed upon. What if Russia took pains to camouflage responsibility for a nuclear attack, or at least provided sufficient deniability for smaller, hesitant NATO members to absolve themselves of their collective duty? In such a scenario, the scientific ability to remove doubt about Russia’s guilt may be a significant factor in maintaining NATO’s unity.

Of course, imagining clandestine nuclear use scenarios can easily become fanciful, and the probability of such events should not be overstated. Some scholars, for example, have considered the possibility that terrorists could use nuclear weapons to precipitate a war between two nuclear-armed adversaries—a prospect in which exculpatory scientific data would be especially valuable. Nonetheless, even overly imaginative scenarios illustrate the remarkable diversity of contingencies to which nuclear forensics may be applied.

Forensics in Resolving Accidental Unattributed Nuclear Events. Although unmasking nuclear villainy certainly makes for more dramatic analysis, there are many conceivable, and arguably more likely, scenarios in which nuclear forensics may be brought to bear in response to nuclear accidents or other unattributed nuclear events, some with potentially enormous ramifications. Among the most consequential of these is the possibility of an accidental nuclear detonation, particularly in a troubled, conflict-prone region. Indeed, a 2010 study by the National Academies of Science noted that immediately following a nuclear detonation, one of the first questions leaders might ask is, “Was it ours?” However, such introspection should not be assumed in regions where one or more rival states possess nuclear weapons and are locked in a persistent state of low-grade hostility. Following an accidental nuclear detonation on the Indian subcontinent, one can easily imagine the leaders of Pakistan or India reflexively concluding that their nation was under attack and ordering nuclear retaliation. Such an event could precipitate a nuclear exchange that rapid, unambiguous scientific analysis might be able to forestall, staving off a tragedy of immense proportions.