In the aftermath of June 2016 EU Referendum result the majority of attention has focused on what might be future economic relationship between the UK and the EU and the prospects for the UK’s trade relationships with third countries once outside the EU. None of the proposed models for the future trade policy relationship between the UK and the EU (for example, membership of the European Economic Area or a Free Trade agreement) come with a defined foreign and security policy relationship. Further, Article 50 of the Treaty on European Union, providing for the exit of a member state from the EU, does not offer roadmap to a new status of foreign, security and defence policy relationship between the EU and its exiting partner.

Security and defence policy gives effects to the broader foreign policy aims and ambitions for a state. For the UK the EU has been a centrepiece of foreign policy since accession in 1973. Consequently exiting the EU presents the prospect of a major rethink in the aims and ambitions for Britain’s place in the world and has implications for the conduct of British diplomacy and will impinge on security and defence policy. This broader rethink on Britain’s place in the world hasn’t yet started.

In an article I just published in the NIESR November Review I examined one strand of the UK’s present and foreign and security policy which is currently conducted through the EU. In evaluating the current EU-UK interrelationship in the fields of the EU’s Common Foreign and Security Policy (CFSP) and Common Security and Defence Policy (CSDP) it is clear that the costs and benefits of Brexit are differential for both sides.

With the UK’s departure the EU will be losing a major diplomatic player and one of its two, alongside France, most militarily capable member states. For the EU the most immediate impact of Brexit on the foreign, security and defence policy area has already been to give impetus to ideas on reforming EU defence policy and which have been in circulation for some time. A set of proposals have been made for deepening of the existing defence collaboration between the EU’s other member states in the absence of the UK.

The relatively under-developed and intergovernmental nature of EU defence policy does mean that the impact for the UK in departing from the EU’s existing policy in this area would be marginal. The UK would, however, have a greatly diminished capacity for shaping the future agenda for EU defence policy and EU policy may develop in a direction that the UK views as contrary to its own interest’s post-Brexit.

Exiting the EU’s foreign and security policy would appear to carry more significant costs for the UK. The CFSP currently provides significant efficiencies for the UK in addressing a wide range of foreign policy and security issues, via a multilateral format, with twenty seven other European countries. It allows the UK to amplify national foreign and security policy interests by having these translated into collective positions held by twenty eight countries.

In embarking on the process of exiting the EU future arrangements for cooperation in the areas foreign, security and defence policy will need to be negotiated. Both the UK and the EU and its member states will need to take a view as to the characteristics of their future framework for cooperation. The UK will need to determine the degree to which it wishes to seek autonomy from the EU in foreign and security policy-making processes and the extent to which it might envisage national policies diverging from the portfolio of existing EU policies. Shared borders and a common neighbourhood will dictate the need for working in partnership. The foreign, security and defence policy component of the future EU-UK relationship, therefore, should represent the most straightforward aspect of the future relationship that is to be negotiated. A key principle for negotiation should be to ensure that the UK’s diplomatic and military capabilities are broadly aligned with the EU’s foreign and security policies and allow for synchronised policy and action.

A key question for the UK and the EU in the foreign, security and defence policy area is the degree to which both sides seek a relationship that sees the UK remaining coordinated with the existing EU decision-making and implementation machinery. For its part, the UK is unlikely to be content with a relationship of just adopting pre-determined EU policies (on which it has not had an influence in framing). However, as is the case with many other aspects of the future relationship with the EU, an issue will also be the extent to which the UK seeks to establish policy differences to demonstrate where advantages come from its new post-Brexit status.

Richard G. Whitman is senior fellow at The UK in a Changing Europe and visiting senior fellow at Chatham House