Commentators have rushed to weigh in on the political and economic implications of the Brexit referendum. But the potential security effects are just as important. At risk are operational matters such as data and intelligence sharing. But also in question is something more fundamental: the relationships that allow security services to live and breathe. The United Kingdom, EU, and other partners will now have to redefine their security and intelligence relationships.

Such negotiations will take time. And before taking a seat at the table, all parties would be well served to think carefully through some of the critical strategic and tactical questions that will have to be addressed, even as they bear in mind that fissures among EU member states’ law enforcement and intelligence communities predate the referendum and that adversaries are likely to seize any fragmentation as an opportunity to test resolve.

Europe’s fracturing gives Russia a chance to push the envelope, as it has been doing in recent years in Ukraine and elsewhere. For months now, Russia has been testing the continental and transatlantic alliance by bringing difficult and potentially divisive issues to the fore. Recent Russian exercises, deployments, and rhetoric have seemed intended to probe the depth of the EU’s and NATO’s commitment to securing their borders.

Russian fighter craft have buzzed the edges of NATO airspace for months, trying to ascertain the limits of allied discipline and restraint. Beneath the sea, Russian submarines have been “aggressively operating near” the undersea cables that are instrumental to the functioning of the U.S. military and the world economy. In cyberspace, meanwhile, Russia has tried to destabilize foes through propaganda, including by depicting Germany as “a society in chaos because of migration.” And pulling no punches, within hours of the British referendum, the mayor of Moscow stated that “without Great Britain in the EU, no one will so zealously defend the sanctions against us.” In short, Brexit is effectively a gift to Russia, and it will likely keep on giving.

The Islamic State (also known as ISIS), too, has looked to weaken European unity and destabilize the West. In one issue of its newsletter, the group boasted that the Paris attack could lead to the “the weakening of European cohesion, including demands to repeal the Schengen Agreement.” To this end, it is alarming that the planning for the latest ISIS attacks in Belgium and France spanned so many EU countries. The perpetrators shared information on everything from concepts to capabilities through complex networks that took advantage of gaps in information, intelligence, and cooperation, within and among allied security services. If anything, these attacks showed the need for Europe to come closer together, not further apart, via Brexit or otherwise.

EU entities such as Europol are tasked with Europe’s security and intelligence portfolio and will lose the substantial assets, tangible and not, that the United Kingdom has brought to the table. The country’s highly capable security and intelligence services have helped power EU efforts in this area, and even if the partners reach alternate arrangements (for access to data and so on), the United Kingdom will lose the ability to lead and influence from within. There is also the EU’s relationship with NATO to keep in mind. It has always been a challenge to avoid duplication and inefficiency between the EU and NATO. But the potential for divergence between the two entities could be magnified now that the United Kingdom is no longer around to bridge the breach. Taking a hardheaded approach to threats—including those in the newest domain, cyberspace—the United Kingdom has worked in the past to focus European minds and resources on the most pressing issues, in a way that makes use of the complementarity between these security architectures. Whether this process will continue in practice, rhetoric aside, remains to be seen.

Historically, the United Kingdom has also acted as linchpin between Europe and the United States, cementing the security bond across the pond. Although some of the luster has worn off the special relationship, the fact remains that the United Kingdom has long served as a touchstone for the United States in its dealings with Europe writ large, and it remains the United States’ closest European ally. With both countries now outside the EU bloc, and in the absence of a principal long-standing interlocutor, the United States could well find it harder to make its case to Europe on issues of critical importance. From the imposition of economic sanctions on Russia and Iran to the designation of Foreign Terrorist Organizations, the United States was instrumental in getting to the goal when differences within the EU existed.

At a more tactical level, Brexit could harm efforts to combat terrorism and organized crime. In both areas, data sharing is the coin of the realm. At the working level, law enforcement and intelligence authorities in different countries may well have robust and productive cross-border relationships, based on mutual trust and respect built up over time. But these ties can take policing only so far in the absence of formal agreements to underpin and authorize such exchanges. To maintain its present level of access, the United Kingdom would have to negotiate a host of bilateral arrangements with remaining EU member states, each with its own disposition, views, and possible sticking points on the matter.

Brexit comes at a time when European security is already under threat. Both state and nonstate actors are brazenly challenging Europe and it allies. Substantial uncertainty lies ahead for the United Kingdom, for the EU, and for the relationship of both to the United States. Reaching a new modus vivendi that maximizes safety and minimizes divisions is in the interest of all of the parties. Those responsible for defining, calibrating, and implementing this new equilibrium surely know as much. The challenge, however, will be to insulate and protect the law enforcement and intelligence domains from the political bluster and positioning that will surely accompany the negotiations.

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