LONDON — As Britain prepares to withdraw from the European Union, it is cultivating closer military ties with the Continent.

The one thing that could scupper these efforts is what's motivating them: Brexit. If talks on Britain's departure from the EU descend into acrimony, or no final deal is reached, it will be hard for either side to get closer on the military front.

For now, Britain is pushing ahead on security, even as Brexit negotiators struggle to make progress. In keeping with Prime Minister Theresa May's mantra that Britain is leaving the EU but not Europe, the U.K. is sending strong signals it remains committed to the defense of the Continent. Britain's chief of the defense staff was recently elected as the head of NATO's military committee. British Cabinet ministers have floated the idea of David Cameron taking over as the Atlantic alliance's secretary-general.

Britain is also working to boost bilateral defense cooperation with Continental allies — above all France, Europe's other military heavyweight. At a Franco-British summit in January intended to be higher in ambition and wider in scope than previous such events, officials will discuss how to turn the Combined Joint Expeditionary Force the two countries have established over the past few years into a major battle-ready unit.

Having long been cool on common EU defense initiatives, which they saw as undermining NATO, British officials have warmed to the idea. They say they want a special military relationship with the bloc after Brexit. That could enable the U.K. to keep taking part in EU military missions, as long as it was involved in operational planning. Multiple EU nations are also pushing to find ways to keep London as involved as possible in the bloc's military endeavors, although some EU officials say Britain could not be involved in military decision-making as a non-member.

“Britain’s commitment to European defense is not just in the interests of the EU but also of the U.K." — Tom Tugendhat, Conservative chairman of the U.K. Foreign Affairs Select Committee

For European allies, Britain has a lot to offer in military terms. It is the only European power to extend its nuclear umbrella over other NATO members. It has troops in the Baltics as part of NATO efforts to deter Russian aggression. It leads the EU’s Atalanta counter-piracy mission off the coast of Somalia and plays a significant part in Operation Sophia, which aims to counter migrant trafficking in the Mediterranean Sea.

There would also be benefits for London in a new special military relationship with the EU. Brussels is ramping up plans for joint spending on defense procurement and British companies would like a slice of that pie.

'Infection risk'

EU officials are waiting to see how the Brexit process plays out. If cross-Channel relations sour, there will be little appetite in Brussels or major capitals for developing a new defense partnership with the U.K., despite its military clout. The feeling is much the same on the British side.

“Politically, in these circumstances, it will be much more difficult for us on security and defense cooperation with the EU — it does risk being infected,” one U.K. defense official said.

Though Donald Trump is hard to predict, Britain’s closest defense and security partner will almost certainly remain the United States, U.K. officials say. The "five-eyes" intelligence-sharing network between the U.S., U.K., Canada, Australia and New Zealand is unparalleled in its depth. British ministers also hope a post-Brexit U.S.-U.K. free trade deal could further open the United States’ defense market to British firms like BAE Systems.

But Britain's also pushing to retain influence in Europe as it pulls out of the EU — and building defense ties is central to that effort, particularly with France.

“Britain’s commitment to European defense is not just in the interests of the EU but also of the U.K.," said Tom Tugendhat, the Conservative chairman of the Foreign Affairs Select Committee in the House of Commons and a former British Army officer.

"I am sure that the U.K. and our EU partners, none more than France, appreciate the British contribution to our shared interest," he said. "We are part of the European defense arrangements because of our geography and history and we will remain strong allies for generations.”

The Franco-British expeditionary force is the key to plans to move closer to Paris. The force was established in the 2010 Lancaster House agreement, signed by Cameron, early in his first term as British prime minister, and France's then-President Nicolas Sarkozy.

It is not a permanent standing force but is made up of land, naval and air force elements that can come together to carry out joint missions on the orders of the British and French governments. Five thousand troops from the combined force took part last year in the first major showcase of its strength, Exercise Griffin Strike in the south of England.

Bienvenue! to Brigadier Hervé Bizeul as Dep Comd 1 (UK) Division from Maj Gen Hill and the Division @armeedeterre pic.twitter.com/1lZxrNhlsO — 1 (UK) Division (@1UKDivision) September 13, 2016

The force is still in the relatively early stages of development, set up for humanitarian missions and extracting nationals from dangerous situations. But officials from both countries want to develop it into a force capable of being sent into joint combat operations.

Officials aim to agree the next steps in the joint force’s capability as part of wider moves to “refresh” the Lancaster House agreement, according to a U.K. official familiar with the plan.

London and Paris envisage the force being used in regions like the Sahel and North Africa, where both countries have a strategic interest.

According to officials in Paris, France sees Britain as the only serious military partner willing and able to take on combat missions outside Europe. Germany, although now committed to increasing military spending, remains reluctant to play an assertive military role given its militaristic past.

In one of several signs of increasing cooperation between French and British forces, French Brigadier Hervé Bizeul last year took up a post as deputy commander of the British Army’s First Division.

At the moment, as part of anti-ISIS air strikes over Syria and Iraq, a Royal Air Force pilot is embedded with a French Rafale fighter jet squadron while a French pilot carries out missions with an RAF Typhoon squadron.

Later this month, senior officers from both countries will take part in a headquarters-level exercise in Paris for the joint expeditionary force.

But Brexit casts a shadow over this new entente cordial. Even if the EU and Britain reach a Brexit deal but it's on the basic side, without clarity on the status of EU workers in Britain and vice versa, that could jeopardize joint projects in the defense and aerospace industries.

And a disorderly Brexit could call into question transfers of classified information between the EU and Britain.

"All of this needs to be examined in detail, point by point, in terms of Brexit's impact. We're a long way from settling these issues," Jean-Pierre Maulny, deputy director of the French International for International and Strategic Affairs in Paris.

Reliability reconsidered

France also has other concerns about Britain's reliability as a military ally. When the joint force was in its infancy, London and Paris seemed on the same page militarily, both taking part in the bombing of Libya in 2011

Enthusiasm for joint operations has since cooled. Britain hardly participated in France's anti-terrorism operation in Mali, with other EU partners providing crucial logistical support.

Maulny said the British parliament's vote in 2013 not to back Cameron's plan to attack Syrian President Bashar al-Assad’s regime in Damascus over his use of chemical weapons also rang alarm bells in Paris.

"Bilateral is still the name of the game when it comes to security and defense” — Jan Techau, security expert

"For France, that vote fixed a limit — not in terms of having a partnership with Britain — but in terms of being able to say 'we'll have the same operational priorities as the Brits,'" said Maulny. "That's just not the case. If you take Africa, for example, classically that's not Britain's strategic area."

He added: "Right now, from a strictly political perspective, we have more of an interest to do things on the European level."

That European level has developed rapidly since Britain voted to leave the EU last year, although it still remains embryonic. Twenty-three countries declared last week that they would be part of the EU's new military coordination pact, known as Permanent Structured Cooperation (PESCO), and more are expected to join before it is launched next month.

But having such a large number of nations on board, often with very different ideas about the use of military power, also presents major challenges and is likely to severely limit the pact's effectiveness.

PESCO is mostly “procedural” and strong bilateral ties with the British military would continue to appeal to Paris, said Jan Techau, a security expert who heads the Richard C. Holbrooke Forum at the American Academy in Berlin, described

In the French view, the EU process is “too tedious and slow,” he said. “Bilateral is still the name of the game when it comes to security and defense.”

“The French are absolutely willing to deal with the U.K. outside of existing alliances and form a coalition of the willing if necessary,” added Techau, who served as an official in Germany's defense ministry. “The culture and mindset of the British and French is much closer on security than with Germany. They know they are the only two they can rely on. Germany is the outlier.”

Matthew Karnitschnig in Berlin contributed reporting.

This article is part of a series: The New Transatlantic Order.