Nigel Farage, left, head of the U.K. Independence Party and a backer of a British exit from the European Union, with and European Commission President Jean-Claude Juncker in Brussels last week. (Yves Herman/Reuters)

The European Union commands no army, navy or air force. It doesn’t run a spy service, either, and its Brussels-based bureaucrats can’t make decisions about war and peace.

Yet when Britain votes June 23 on whether to leave the world’s largest economic and political bloc, the pillars of Western security could be on the line.

A British decision to exit the 28-member union, experts say, could have disastrous repercussions for a U.S.-led post-Cold War order that relies increasingly on a united Europe to deter adversaries through economic sanctions, to pool intelligence about trans-national threats and to supply the diplomatic muscle to help contain conflicts.

While a British departure would not preclude the E.U. from those roles, the loss of a nation with the world’s fifth-largest economy and defense budget would undoubtedly blunt Europe’s punch. And it could presage a broader disintegration at a time when Europe’s adversaries — from Russia to the Islamic State — already sense a level of vulnerability on the continent unlike any seen in decades.

“This is a huge call,” said Robin Niblett, director of the London-based think tank Chatham House. “The U.K. would be leaving the E.U. at a particularly dangerous moment in international relations. That can’t be a good thing.”

But ironically, security fears may ultimately help those arguing for “out” more than they do backers of the call to remain “in.”

Even as Prime Minister David Cameron has put geopolitical risk at the center of his campaign for Britain to stick with the E.U., advocates of a British exit — known as “Brexit” — have already begun to bombard British voters with a much simpler and more straightforward message: Europe is a hot mess, and the only way for Britain to stay safe is to dig a wider moat between itself and the continent.

“Up to 5,000 jihadists in Europe after training with Islamic State,” Nigel Farage, leader of the U.K. Independence Party and one of the country’s most prominent champions of Brexit, recently tweeted. “EU open borders make us less safe.”

The tweet was linked to a report in which the director of Europol, the E.U.’s policing agency, warned that thousands of Europeans had received military training in Syria or Iraq and subsequently returned to the continent, contributing to “the highest terror threat in more than ten years.”

But the official, Rob Wainwright, has been outspoken in recent days arguing that Brexit would not insulate Britain from that danger. In fact, the former intelligence officer has said it would heighten the risk by limiting British involvement in E.U.-wide counterterrorism cooperation that has built up over decades.

“If you take away that infrastructure that [the British police] have helped to design over the past 40 years, it would make the United Kingdom’s job harder to protect citizens from terror,” he told reporters at Europol headquarters in The Hague.

Experts say that cooperation will not disappear if Britain ends up backing out. But it would have to be rebuilt. In the meantime, Brexit proponents have struggled to explain why Britain would be safer if it pulled up the drawbridge to Europe.

Iain Duncan Smith, a senior minister who defied Cameron by advocating for a British exit, recently told the BBC that free-movement policies on the continent left Britain vulnerable to attack.

“This open border does not allow us to check and control people that may come and spend time,” he said. “We’ve seen what happened in Paris.”

Left unsaid, however, is that Britain already does extensive checks along its borders. Unlike much of continental Europe, it has opted out of the passport-free Schengen zone, meaning that people do not get into the country unless the government decides to let them.

Also left unspoken by the “out” camp is the fact that Britain’s biggest security threat probably comes from within — not from continental Europe — in the form of homegrown extremists.

Still, the referendum — which polls indicate could go either way — is likely to play out against a European backdrop that is far from reassuring. Cameron had given himself until the end of 2017 to hold the vote. But he opted to stage it in June, reasoning that a delay would give the “out” campaign more time to make its case.

But that could prove a mistake, creating the potential for Britain to make its choice just as summer migrant and refugee flows onto the continent hit their peak. With less than four months to go before the vote, Europe still has no coherent plan for how to deal with an issue that has become intertwined in many voters’ minds with fears of terrorism.

“It’s terrible timing,” said Jan Techau, director of Carnegie Europe.

Techau said Cameron can still win the argument over security. But the prime minister will be challenged to focus British voters’ attention on the broader picture of collaboratively boosting Europe’s external border controls.

“You have to make the long-term case,” said Techau, a former official in Germany’s Defense Ministry. “If he lets this thing get carried away by short-term fears, he will lose.”

To bolster his argument, Cameron has enlisted former military commanders and intelligence chiefs. In an open letter to Britain’s Daily Telegraph newspaper, a dozen retired leaders of the armed forces argued that the country faces an array of grave threats and that E.U. membership is essential to combating them.

“Britain will have to confront these challenges whether it is inside or outside the E.U.,” they wrote. “But within the E.U., we are stronger. Inside it, we can continue to collaborate closely with our European allies, just as we did when we helped to force the Iranians to the negotiating table through E.U.-wide sanctions, or made sure that [Russian President Vladimir] Putin would pay a price for his aggression in Ukraine.”

Brexit backers scoff at those claims. Even if Britain leaves the E.U., they argue, the country will remain in NATO — the military alliance whose primary purpose is to defend Europe.

The E.U., by contrast, has never had a defense mission, with opponents of deeper integration — including Britain — beating back attempts through the years to field a European army.

The original purpose of the union, born from the ashes of World War II, was economic. But the E.U.’s architects always had something bigger in mind: the idea that shared financial incentives could end Europe’s ignominious status as the world’s primary source of violence and instability.

As peace has largely held on the continent in recent decades, the E.U. has become much more of a player in global affairs. Its ability to enact sanctions, pursue diplomacy and mete out development assistance has proven particularly useful in addressing the sort of asymmetrical, 21st-century security threats that a traditional military alliance such as NATO has struggled to confront.

That has made the union an ever more important partner for the United States, especially as direct U.S. involvement in Europe has receded.

But now, a Brexit could erode both E.U. and British influence. For the union, it would mean the loss of a diplomatic heavyweight and a possible open door to future defections as anti-E.U. forces gain ground throughout the continent. For Britain, it would mean cutting itself off from its primary engine of foreign policy gravitas.

And for Washington, it could mean a sudden absence of robust allies across the Atlantic.

“An unstable E.U. makes a bad partner to the U.S.,” said Chatham House’s Niblett.

He said Washington regards the British push for an exit as “self-indulgent” and “strategically irresponsible.”

“The sense from the U.S. is: ‘You cozy up to China and you split the trans-Atlantic community. If you’re not thinking strategically, then you’re not a good ally for us,’ ” Niblett said.

That’s why President Obama has been outspoken in backing “in.” All of Britain’s other major allies have, as well.

But not Russia. Moscow has made little secret that it would welcome the sort of European unraveling that a British exit could trigger.

“A divided Europe is one in which Russia is a much stronger actor,” said Ian Kearns, director of the European Leadership Network. “Putin would be delighted by Brexit.”

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