In his address to the Lowy Institute last week, Chatham House director Dr Robin Niblett identified security as perhaps the most surprising element of the Brexit debate. In the counter-terrorism context, the debate was turbo-charged by the terrorist attacks in Brussels just over two months ago. And more specifically, by the comments of Sir Richard Dearlove, a former head of MI6, who claimed that while the costs of Brexit from a national security perspective would be low, it would deliver critical security benefits.

‘Out’ campaigners have expanded on these comments, claiming that the net effect of Brexit would be to make the UK safer from the threat of terrorism. They have three main arguments:

Firstly, that the UK is primarily reliant on the Five Eyes intelligence partnership for its counter-terrorism intelligence. And as Europe’s leader on security and intelligence matters, the UK gives the EU much more than it gets back. In the event of an ‘out’ vote, the Five Eyes would endure, while the EU as a whole (and EU nations bilaterally) would be desperate to re-establish partnerships with the UK.

Secondly, they point to the security flaws that contributed to the Paris and Brussels attacks, in particular that some of the attackers were able to return from the Middle East and cross European borders without being identified and stopped. Brexit would close the ‘open door’ that allows would-be terrorists to enter the UK.

Finally, EU membership allows European courts to rule on UK security matters. This — out campaigners argue — makes it more difficult for the UK to refuse entry to EU citizens, even when authorities believe the individual is linked to terrorism. And once those individuals are resident in the UK, it is also more difficult for authorities to remove them.

Of these three arguments, the first has most merit. The UK does have the strongest intelligence and law enforcement agencies in the EU, and is a net contributor when it comes to the fight against terrorism, thanks in no small part to the intelligence and capabilities it derives from the Five Eyes partnership.

In the event of Brexit, the rest of the EU would undoubtedly be keen for this to continue. And key EU institutions have the ability to do so. Europol, increasingly central to Europe-wide counter-terrorism efforts, has operational level agreements with non-EU nations, including the US and Australia. Considering the UK’s outsized contribution to Europol, a similar agreement would surely be implemented quickly.

However, continuing these relationships is not the same as improving them.

In an inter-connected Europe, the success of collective counter-terrorism efforts will be determined by the lowest common denominator. The more the UK can use its experience and expertise to drive reform and improve the performance of fellow EU agencies, the better. Could the UK take the driving seat in much-needed efforts to improve EU-wide counter-terrorism capability as a partner rather than member?

And counter-terrorism and intelligence are different from the UK's other relationships with the EU. Benefits cannot be evaluated by comparing the size of incomings and outgoings. Despite the UK’s impressive capabilities and partnerships, the EU framework gives the UK access to pieces of the puzzle that it would otherwise struggle to obtain. In other words, the UK’s intelligence contribution to the rest of the EU is not charitable; it helps keep the UK safer too.

Turning to the other two ‘out’ arguments, there is little to suggest that concerns about the UK’s inability to prevent the movement of EU national terrorists into the UK are grounded in reality.

Take for example the Paris and Brussels attacks, a potential template for future ISIS-directed attacks in Europe. The attack cell used fake or stolen passports to return to Europe from the Middle East. And those that had remained within the EU had, despite numerous opportunities, not been identified as potential terrorists.

While this does not paint a rosy picture of EU border security and intelligence capabilities, it is also not the scenario described by ‘out’ campaigners; known terrorists using their own EU passports to pass through border controls against the authorities’ wishes. And the reality remains that while the threat to the UK from EU nationals is real, it is insignificant in comparison to the threat to the UK from UK nationals.

Whether the UK votes in or out in one month's time, this threat will endure. But if terrorist attackers will be primarily (but not exclusively) local, their connections — like those in Paris and Brussels — are likely to be transnational.

Brexit would not deal the UK’s counter-terrorism relationships with the EU a fatal blow. But it would disrupt them at a time where Europe’s counter-terrorism agencies are under strain, and limit the UK’s ability to shape a Europe-wide counter-terrorism strategy in the years ahead. It is hard to see how Brexit would reduce the terrorist threat to the UK.

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