It should come as no surprise that David Cameron and the campaign for the UK to remain in the EU have argued that Britain’s continued membership of the EU will benefit the UK’s security. Britain’s EU membership has always had a security side to it. Membership has never been entirely about trade or jobs. And whatever the result of the referendum, the security relationship with the EU will remain vital for the UK.

Those campaigning for the UK to leave the EU do not dispute that relations with the EU, along with European security in general, are not of central concern for the UK. For them, however, not only does the EU make the UK and Europe less safe (for example by allowing free movement of criminals and terrorists), but putting international relations ahead of domestic politics, especially economic and democratic needs, weakens Europe’s nation states and their ability to combat the full range of problems facing them.

The UK government itself is clear that traditional security threats to the UK do not figure as highly as they once did, although they have certainly not disappeared. As successive national security strategies and strategic security and defence reviews have made clear, security challenges to the UK range from nuclear war to environmental disasters, with many not recognising the borders of nation states. While British governments are often reticent to admit to the EU playing a positive role in security cooperation, or admit that the UK has led in such cooperation, there is no denying that tacking these security challenges has been an important part in UK-EU relations.

From the start, European integration has had a security side to it, whether this be about managing the reintegration into Europe of West Germany and later a united Germany, or integrating former Eastern European Communist or Southern European Fascist states into a liberal European mainstream. The EU itself has not been sufficient to keep the peace. The argument by some Eurosceptics that it has been NATO that has kept the peace in Europe is as selective a reading of history as any pro-European that claims it has all been down to the EU. Nevertheless, for successive British governments, especially at the height of Britain’s retreat from empire in the 1960s, Britain’s declining ability to shape the world and the security challenges it posed meant membership of the European Economic Community and later EU was a necessary step for both British, European and transatlantic security cooperation.

Today, Cameron and others in the Remain campaign trumpet Britain’s ability to use the EU to boost Britain’s still substantial but reduced economic and military capabilities to give it the full range of tools and opportunities to face the full range of security challenges. Whether in facing a newly assertive Russia, dealing with Iran or tacking climate change, the UK’s EU membership has been cited as crucial to allowing the UK to shape its security. This is not to argue that EU cooperation is without its problems. Finding consensus amongst 28 states is never easy, even when you have the institutions and familiarity that EU membership offers.

The referendum poses the question of what options the UK has beyond working through or with the EU. Talk of the UK rebuilding the Commonwealth is often oblivious to how strategically marginal that organisation can appear to most of its members. Britain’s NATO membership will remain important to it and Europe. NATO, however, is not the whole story when it comes to European security and the UK should not bet on the alliance being as solid as it was once. Close bilateral relations with the likes of the USA or France will remain, not least in the military fields. But this should not blind us to how the US and European states will continue to cooperate through a range of EU-US forums and that this could leave Britain outside the EU as something of an awkward in-betweener in the transatlantic relationship.

The US in particular is aware of the wider implications of a Brexit for Europe’s geopolitics. As Condoleezza Rice, former Bush Administration Secretary of State and National Security Advisor, told Chatham House: ‘It is a very different Europe if it is a continental one’. Accusations that the UK is a US ‘Trojan Horse’, sent to weaken EU cooperation from within, display an ignorance of how the USA – a European power since 1945 – has had a hand in European integration and security from the very start and doesn’t wholly depend on the UK for relations with the rest of the EU.

Britain’s departure from the EU could change the Union in any number of ways, with far-reaching geopolitical implications that will shape Europe’s security. That some Leave campaigners such as Michael Gove have spoken of Britain setting an example that could lead other states to follow gives a hint to the wider geopolitical changes to Europe some Eurosceptics hope a Brexit will bring about.

Such disintegration or radical reconfiguration of the EU has not been set out or anlaysed in any document or strategy. As such this half-baked approach risks a dangerous case of strategic overreach by the UK. Any changes to Europe would rely less on Britain’s willingness to offer the ways and means of managing such changes and more on encouraging a collapse of the EU. Losing control of such a development should not be overlooked. An uncontrolled collapse of the Eurozone, for example, would inflict significant costs on the UK, Europe, the USA and the wider world economy. As HM the Queen warned in June 2015 during a state visit to Germany, Europe’s division is in nobody’s interests.

It is possible that a Brexit will cause the opposite reaction, with an EU rid of ‘an awkward partner’ going forward by unifying further. One of Britain’s longest standing international aims has been to prevent any single power dominating Europe. The EU would be a benign power compared to previous attempts, but such an outcome warrants careful consideration by the UK. Whatever the outcome, the EU’s status quo is unsustainable. The question for those concerned with Europe’s security is whether changes – whether triggered by Brexit of not – can happen in a stable, cooperative way or bring about a collapse into nationalism and parochialism.

The best hope for UK and EU security is that whatever the result of the referendum both sides seek ways of cooperating, if only to rebuild trust. The initiative here will rest with the UK whose recent approach to the EU in the eyes of many elsewhere in Europe has been an unwanted distraction from the many other challenges facing the EU. A UK in search of cooperation with its European neighbours will find it can offer little to help with the problems in the Eurozone or Schengen. Instead, whether in or out of the EU, it will find itself playing its only trump card: security and defence cooperation.

Whether in facing a resurgent Russia – and therefore the continued relevance of hard power in European and international relations – or the more nuanced but potentially far more catastrophic global environmental challenges, the UK and EU could find common ways forward that help both sides realise their mutual interdependence. The UK and the rest of the EU must appreciate that instability or division in Europe limits their options to shape security in the wider world.

Last, but not least, we should not overlook that if the first priority of any state is its own survival then alarm bells should be ringing about how the referendum could lead to the end or weakening of the UK itself. The vote could throw into doubt the place in the Union of Scotland and Northern Ireland, both bringing with them a host of traditional security concerns whether they be terrorist related or the future of the UK’s nuclear weapons. Tensions within England should not be overlooked. Immigration and economic changes have made London the UK’s undiscovered country, much to the chagrin of some elsewhere in England and Britain who feel a Europeanised and globalised capital city has left them behind. Arguments over identity, immigration, radicalisation, race, equality and standards of living rumble beneath the surface of the EU referendum. A reminder, if any were needed, of how sustaining the UK’s own stability and security will be no easy challenge after the 23 June.

Dr Tim Oliver is the Dahrendorf Fellow for Europe–North American relations at the London School of Economics and Political Science. He is also a non-resident fellow at the Center for Transatlantic Relations of the Johns Hopkins University Paul H. Nitze School of Advanced International Studies, Washington DC. He has worked in the House of Lords, the European Parliament, the German Institute for International and Security Affairs (SWP), RAND Corporation, and taught at the Royal Military Academy Sandhurst, the London School of Economics and University College London.