Whether Brexit happens or whether it does not, one thing will not change. Britain will need a defence strategy for the future that better reflects its real place in the world. Geographically, Britain’s place is going nowhere, Brexit or no Brexit. Geopolitically, the world is changing. As China’s power rises, the US turns isolationist under a dangerous president, terrorist and cyber-threats continue and nuclear arms proliferate. Britain’s defence strategy needs to adapt and keep pace.

Prime ministers and defence secretaries still talk as if Britain is a global power with post-imperial reach, able to deploy a sweeping range of armed forces and weaponry in support of allies, principally the United States, from the Irish Sea to the Pacific. Most of Britain’s wars and deployments since 1945 have been made on that basis, with some exceptions like the missions in Northern Ireland and the Balkans. But this is not sustainable on the scale of the past, either politically or financially.

Although there were big cuts following the 2010 defence review, a funding gap of at least £20bn remains over the next decade. As things stand, the plan is for reduced armed forces, fighting ships scrapped, planes retired early and units merged; meanwhile we maintain renewed nuclear weapons and two new aircraft carriers. Instead of having spending that fits the strategy, Britain increasingly has a strategy that fits the spending.

When politicians in defence debates insist on some arbitrary figure of spending instead – it used to be 2% of GDP, but 3% is beginning to be commonly invoked – they simply compound that error. There may well be a case for Britain spending more than it does on defence, but the justification for it must be strategic, not based on the fetish of a particular figure. Defence spending should neither be arbitrarily high nor arbitrarily low.

The former chief of the defence staff General Lord Richards said last week that Britain needs to decide what sort of country it wishes to be after Brexit. It could be outward looking and strong, he said. Or it could be inward looking and no longer a premier military power. That is a false choice. The choice is not between outward and inward, Premier League or Second Division. It is between remaining a global military power and focusing on being a regional one.

Such choices are never as neat and tidy as theorists and generals pretend. Distilling a lifetime of military thinking, Lawrence Freedman’s book The Future of War makes abundantly clear that defence is a deadly serious business about which some scepticism regarding future threats is always wise. Britain cannot make itself immune to global threats from the Middle East, North Korea or climate change by an act of will. There will always be calls of emergency, solidarity, and wider national and allied interest.

But Britain is a European nation. The threats it faces – from Russia, terror, cyber-attack, climate change, instability in the Middle East and north Africa among them – are all shared with our neighbours and allies. Whether or not the UK remains in the EU, Britain remains in Europe. That means Britain should match its defence and security priorities to this reality. It should work with, not against, the response from the EU and our European Nato partners.

This week President Macron visits Britain for an Anglo-French summit. Thursday’s meeting with Theresa May is not exclusively about defence, but its location at Sandhurst army college is resonant. As the former national security adviser and Foreign Office chief Peter Ricketts says in a briefing paper for the Royal United Services Institute, Britain and France are Europe’s global players and its defence heavyweights. The two have much in common – including cooperation built up over the past 20 years on nuclear, the joint expeditionary force, equipment, intelligence and migration. It is in our mutual interest not to drift apart because of Brexit or the impact of Donald Trump. But it is also in our mutual interest to deepen defence cooperation and not allow issues like the proposed European defence fund to be sticking points. The defence relationship with France needs more input and imagination, not less. It must be at the very centre of what should be Britain’s Europe-focused future defence thinking, whether we are inside or outside the EU.