When the Proliferation Security Initiative (PSI) was founded in 2003, it was very much a creature of the United States. Specifically, it was a creature of then Under Secretary of Arms Control and International Security John Bolton. The PSI, according to Bolton, was “not an organization, but an activity” in the service of the George W. Bush administration’s goal of creating “web of counterproliferation partnerships through which proliferators will have difficulty carrying out their trade in WMD and missile-related technology.” Above all else, it was envisioned focusing on interdicting transfers of WMD-related materials. But the PSI has changed since 2003. Its focus has shifted. If the United States is to consider a more aggressive counterproliferation strategy, it must occur beyond the context of the initiative. Over the years, the PSI has grown both in membership and scope. The PSI had eleven participating countries at its first meeting in 2003. Today, over one hundred countries have endorsed its Statement of Interdiction Principles. It now (albeit with pushback from Russia, which joined the PSI in 2004) bolsters not only interdiction capabilities, but also addresses related issues like proliferation finance through the Critical Capabilities & Practices (CCP) tools it distributes to members. While the PSI has thrived as a proliferation-focused forum connecting experts from participating states with peers worldwide, its successes intercepting WMD-related materials (an admittedly nebulous term) are—at least from a public perspective—sparse. The PSI was credited back in 2009 with facilitating at least fifty interdictions but the details of these are not publicly known. Even when specific interdictions have been framed in the context of the PSI, they may have been possible independent of the initiative’s capacity-building efforts. Case in point: a PowerPoint by the Department of State listed both the 2013 interdiction of the DPRK-flagged Chong Chong Gang and 2011’s attempted interdiction of the Belize-flagged Light in the context of the PSI. While the case of the Light fits within the context of the PSI (the vessel was flagged by Belize, a PSI participant who gave the United States permission to board the vessel), the Chong Chong Gang case is only tenuously linked to the initiative. The vessel was not flagged by a PSI participant and the cargo it carried was only related to proliferation in the context of United Nations sanctions on North Korea. In the years following the cases of the Chong Chong Gang and the Light, the UN has vastly increased the scope of its sanctions on North Korea. The range of UN sanctions on the DPRK has, more than ever before, expanded the definition of WMD-related materials. One might expect that this would have been accompanied by more visible enforcement action utilizing the PSI framework. This has not been the case. One reason we haven’t seen flashy interdictions: proponents of the PSI tend to view any means of disrupting the transfer of WMD-related materials as successes, not just interdictions at sea. Moreover, many PSI activities—capacity building programs or support drafting comprehensive WMD legislation, for example—contribute to international security without necessarily leading to clearly identifiable victories. Further frustrating an assessment of the efficacy of the PSI is the fact that notable interdictions, the activity most clearly tied to the PSI, have occurred independent of the initiative. A good example is the April 2018 detention of the Wise Honest, a North Korean vessel caught attempting to export 25,500 tons of North Korean coal for export in violation of UN Security Council resolution 2371 (2017). The events leading to its detention and the ensuing investigation are detailed in the most recent report by the United Nations Panel of Experts on North Korea. Buried in an annex to that report is a letter from Indonesia to the Panel explaining that Indonesia received information on the intended destination of the coal transported by the Wise Honest from an unnamed country. This type of information sharing enables the detection, disruption, and prosecution of proliferation-related activity—all activities the PSI promotes. But the government of Indonesia is notaffiliated with (and historically opposed to) the PSI. This brings into question the efficacy of the PSI as anything other than a discussion forum. Viewing the PSI in this way is not to undercut its value. Having a dedicated forum in which to discuss evolving trends in illicit procurement and the distribution of WMD-related material is incredibly important. As hinted above (and formalized through the PSI’s CCP resources), it allows for the sharing and transfer of expertise in law enforcement, military, legal, and policy approaches to combatting WMD proliferation. Still, separating the present value of that forum from the original concept of the PSI could allow the United States to pursue a more aggressive counterproliferation strategy. Such a strategy would focus on interdictions, either by the United States or partner countries, and carried out in accordance with international law on the high seas or with relevant permissions. The initial push for the PSI placed a strong emphasis on ship boarding agreements designed to facilitate the interdiction of suspect vessels. Because boarding a vessel on the high seas requires the permission of the state whose flag the vessel is claiming (also known as the flag state), securing that permission, or establishing a quick system for receiving that permission, allows for timely interdictions of suspect vessels. To date, the United States has signed eleven of these agreements, the latest of which was formalized in 2010. The UN sanctions regime on North Korea can also be seen as increasing, albeit indirectly, the circumstances in which the United States would have legal authority to interdict a vessel. This is because the sanctions prohibit the registration of DPRK-controlled vessels as well as the re-registration by another country of any vessel de-registered for ties to North Korea. This forces vessels off shipping registries entirely, leaving them stateless and therefore legitimate interdiction targets. The cases in which a vessel may be considered stateless are potentially broader. In investigating the case of the Wise Honest, the Panel discovered that the vessel possessed registration documents for both Sierra Leone and the DPRK. They ruled that this rendered the vessel stateless. If this interpretation of maritime law is accepted, it may also hold true that vessels lying about their nationality of registration are also essentially stateless. This would be incredibly significant: falsely claiming a country’s flag is a common tactic employed by DPRK-linked vessels. Increased latitude to intercept such vessels could be used by naval assets monitoring DPRK illicit activity as part of a more aggressive counterproliferation strategy. Adopting a counterproliferation strategy that emphasizes the interdiction of vessels at sea could increase the risks of inadvertent escalation towards conflict with a state of proliferation concern, like North Korea. Indeed, former National Security Advisor H.R. McMaster and former Secretary of Defense James Mattis reportedly argued over the potential escalation that would result from interdicting North Korean vessels on the high seas. Clearly, it’s a risk that’s being taken seriously. And rightfully so. A war on the Korean peninsula would be catastrophic. Interdicting unflagged vessels or those whose flag states have granted permission for the boarding is inherently different. For starters, it wouldn’t violate the flag state’s sovereignty. It’d be a legal act and one within established international norms. This would, in theory, limit the prospects of escalation to violent conflict let alone one involving the use of WMD capabilities. Still, any interdiction of a vessel potentially operating on behalf of a party of proliferation concern carries significant potential for escalation. If the United States is to move towards a more aggressive counterproliferation strategy, this potential needs to be better understood and mitigation strategies must be considered.