The European Union does have its Common Security and Defence Policy (CSDP) under the umbrella of the Lisbon Treaty of 2009. The policy obliges EU member states to earmark military assets for a Rapid Reaction Force and smaller, more agile Battle Groups.
When it exits from the EU, Britain will cease to work under the CSDP planning obligations and will terminate its financial contributions to the European Defence Agency and related institutions.
One could argue that EU defence posture will be weakened proportionately by the exit of its most potent contributor, but the CSDP is largely a political exercise; its assets are paper commitments and the EU has never deployed its flagged forces except for occasional, small-scale peacekeeping, training, and counter-piracy patrols in Africa and the Balkans.
Britain's departure should make little difference to this modest defence policy.
Also, Britain's exit doesn't spell the end of British defence co-operation with European states. In 2010, Britain negotiated a bilateral arrangement with France, the Lancaster House Agreements, and in 2012 established a joint amphibious force with the Netherlands. Future bilateral co-operation arrangements can be initiated by London as required.
Britain is also a leading member of the 31-state Combined Maritime Forces arrangement based in Bahrain that enlists many European members as well as New Zealand. None of these defence arrangements is under the EU umbrella. Each state will carry on contributing to regional security in co-operation with Britain as long as its national interests are served, Brexit notwithstanding.
And despite the efforts of France to lead a European-based defence alternative, the EU defence institutions are overshadowed by the North Atlantic Treaty Organisation. Nato has been the world's premier alliance since 1949, linking Europe with the US and Canada.
It has grown steadily to its current membership of 28, with Georgia and Ukraine and others applying to join. New Zealand is an associate member.
The EU defence institutions are overshadowed by the North Atlantic Treaty Organisation.
It was Nato that managed to contain the Soviet Union until it collapsed in 1991. It was Nato that took command of liberating Kosovo in 1999, overseeing European troop contingents in Afghanistan after 9/11, and co-ordinating the Libyan intervention in 2011. It is Nato that is currently deploying member forces for exercises in Eastern European states to deter further Russian encroachments.
Britain is perhaps the staunchest member of Nato.
Over the years, London has resisted the growth of EU military institutions partially on the grounds that they would duplicate Nato roles, draw resources away from Nato and undermine the US political will to defend Europe. Brexit will free London from EU and CSDP constraints to work more closely with Nato and the US in military and security policies.
As for the US, Washington has no institutional military link to the European defence institutions.
Instead it works militarily with Europe through the North Atlantic Council and Nato, to which almost all Western European governments apart from Switzerland belong.
The British exit from the EU should have no effect on this alliance, its aims assets or initiatives. And it could actually further enhance US-UK defence co-operation.
On June 27, US Secretary of State John Kerry assured UK Secretary of State for Foreign and Commonwealth Affairs John Hammond that the US-UK "special relationship" would endure unchanged by Brexit.
European and transatlantic counter-terrorism co-operation has improved markedly since 9/11 but still has a long way to go, as illustrated by intelligence-sharing breakdowns between France and Belgium recently, and American complaints about European fastidiousness to do with terrorist data-sharing and prosecution.
Nevertheless, the European Council has appointed a counter-terrorism co-ordinator, and his team liaises with the US State Department's Bureau of Counter-Terrorism and the Secretary of Homeland Security, who in turn work with Interpol and Europol to share intelligence and harmonise policies.
Britain will presumably move out of the EU's formal counter-terrorism institutions but MI-6 will doubtless maintain informal links with both European and US counterparts for mutual advantage, minimising disruption.
Britain's major role in the Five Eyes intelligence network led by the US will continue unaffected.
As for the mass movement of people, even before Brexit Europe's migration management initiatives were fragmented, notably by Britain's opt-outs, Hungarian and Serbian fence-building, and German vacillation between welcome and restraint of the latest surge of migrants.
Ironically, Britain's departure might allow European governments to achieve greater coherence in their collective immigration policies.
So in contrast to the economic sphere where pessimism reigns, the view in the security arena is more sanguine.
Post Brexit, it's business as usual for existing institutions and policies in defence, counter-terrorism and migration management.
