Europe's defence has grabbed headlines this week as world leaders gathered in London for NATO's 70th-anniversary summit -- from the spat over military spending to divisions over the Baltic strategy, and the infamous 'brain dead' controversy.
Yet one pressing issue has received very little attention, even though the clock is ticking: the impact of Brexit on Europe's security.
Uncertainty looms as the UK and the EU are yet to decide on a legal framework for their future defence cooperation just a few weeks before Britain is due to leave the bloc.
"Brexit means Brexit – also when it comes to security and defence," said EU Brexit negotiator Michel Barnier as he was speaking at the annual conference of the European Defence Agency on Friday.
"It will have consequences for our businesses – including our defence industry. This will not be business as usual," Barnier warned.
"In the future, the EU and the United Kingdom will cooperate on different terms than today," Barnier said.
But how this cooperation will take place is far from being clear.
Euronews looks at the future of UK-EU defence cooperation after Brexit and how the departure of the bloc's biggest military power will impact security policies on both sides.
Why is there uncertainty?
"Discussions on the future of defence between the UK and the EU can only happen once Britain has left the bloc," French expert Pierre Haroche of the Institute of Strategic Research of the Military Academy (IRSEM) told Euronews.
The divorce date is currently January 31, 2020, which means negotiations won't start until February.
"The EU-United Kingdom relationship will be a package including not only defence issues but economic, social, trade relations, etc," Haroche continued.
Barnier himself warned these talks would be challenging.
"Once the Withdrawal Agreement is ratified, we have 11 months to deliver the priorities of the trade and security partnership," Barnier said on Friday.
"This will be an enormous challenge. It will require political will and realistic expectations."
"We don't know how far the UK will be willing to go in its cooperation with the EU, because this is politically sensitive for British citizens," Haroche told Euronews.
"What struck me, when I discussed the issue with British people, was how much they depended on what was politically acceptable by their government. Even on issues where we can imagine that the UK has an interest in cooperating with the EU, such as the European Defence Fund which subsidises industrial cooperation - and UK companies are very important in this area - there is always this political uncertainty," Haroche said.
Why does continued cooperation matter?
As NATO is still widely seen as the cornerstone of Europe's defence, Britain's departure from the EU may not seem so significant at first glance. While the UK is due to exit the bloc, it will, of course, remain a full member of the transatlantic alliance.
Furthermore, the UK's contribution to the European Common Security and Defence Policy (CSDP) has been limited -- just 2.3% of total member state personnel contributions, according to the House of Lords EU Committee. From this perspective, the impact of Brexit on EU's defence policy will not be catastrophic.
But for the EU, the departure of one of its strongest defence powers is not without consequences.
As noted by the Institute for government, a think tank, the UK "is one of only two member states possessing ‘full-spectrum’ military capabilities (including a nuclear deterrent), and is one of only six member states meeting the NATO target of spending 2% of gross domestic product on defence."
"The UK also holds a permanent seat on the UN Security Council and has the largest military budget within the EU," the Institute noted.
As the Centre for European Reform put it, "there may be more enthusiasm for European defence co-operation, but less capability" after Brexit.
UK's defence policies may also feel the impact of Brexit in the longer run. “The impact of Brexit on the UK’s military is arguably minimal in the near term. In the longer term, however, the UK’s ability to influence or shape the CSDP agenda going forward will be significantly curtailed,” according to the House of Commons Library.
"The bigger impact will be on the UK’s defence industry: if the UK is outside the EU, and outside the EU Single Market, UK companies will find it much harder to participate in European defence projects, and to access European funds," the Institute for Government wrote.
Therefore, most experts note that it's in both parties' interest to pursue their security cooperation after Brexit.
"The UK and the countries of the EU share the same strategic environment and, by default, the same threats to their peace and security. Historically, pragmatically and geographically, they remain deeply linked from a security and defence perspective, and there is political consensus on the need to nurture this linkage," the European Parliament think tank noted in an April briefing.
How can EU-UK defence cooperation look like after Brexit?
Legally, after leaving the EU, the UK will become a third country to the EU when it comes to foreign and defence policy, Barnier said on Friday.
This means that London will no longer have a role to play in EU decision-making and its contributions to operations will be subjected to the same rules that apply to outsiders.
There are precedents of associating third countries to the EU Common Security and Defence Policy - including Turkey, Russia, Brazil and South Africa.
Areas where the UK and the EU want to cooperate most closely include, according to Barnier, "joining forces in EU-led stabilisation operations in Europe’s neighbourhood," "engaging together on the ground to deliver external action and manage global challenges in a coherent manner; and "cooperating on defence technologies and equipment."
What are the avenues for cooperation outside EU structures?
"Britain's departure from the EU doesn't mean the end of the relations," when it comes to defence cooperation, said Guillaume Lasconjarias, Associate Researcher at the French Institute of International Relations (IFRI).
Beyond cooperation with the EU itself, bilateral cooperation will remain essential, the expert told Euronews, in particular in the framework of the 2010 Lancaster House Treaties between the UK and France.
The bilateral agreements led to unprecedented levels of defence cooperation between the two countries - including a Combined Joint Expeditionary Force, joint maritime taskforce deployments, and joint military training programmes. This will continue after Brexit no matter what.
NATO will also remain a central avenue for Europe's defence cooperation after Brexit.
"Since this decision was made [Brexit], there was a strong comeback and re-engagement of the UK within NATO," Lasconjarias told Euronews.
The new European Intervention Initiative proposed by France in 2017 and set up outside EU structures will also offer renewed opportunities for cooperation after Brexit
"This is an original international organisation (...) there are allies and people outside the EU and this originality means the UK won't be out overnight, it won't be them vs the rest of Europe," Lasconjarias said.
"There is a phase of uncertainty and uncertainty is never a good thing. But there is no anxiety either about what will happen after January 31. That's not where the problem is," the expert continued.
"The key question is, how will we reorganise the order of battles along structures that already exist or that are gaining in importance."