Former London Mayor Boris Johnson speaks at the launch of the Vote Leave bus campaign, in favour of Britain leaving the European Union, in Truro, Britain, May 11, 2016.
Former London Mayor Boris Johnson speaks at the launch of the Vote Leave bus campaign, in favour of Britain leaving the European Union, in Truro, Britain, May 11, 2016.

ROME, ITALY - "Every time Britain has to decide between Europe and the open sea, it is always the open sea we will choose,” Britain’s wartime leader Winston Churchill remarked once to General Charles de Gaulle of France.

On June 23 Churchill’s comment will be tested, when Britain decides whether to remain in Europe or turn to the open sea.

For many Britons eager to exit the European Union, the open sea also means looking west to the United States and falling back on the wartime-forged “special relationship” to enhance British power and to ensure the country outside the European Union doesn’t become isolated or rendered insecure.

But it is unclear how useful Britain would be for Washington as a security and diplomatic partner outside the European Union, even though it would remain a member of the NATO defense pact. Spymasters, generals and politicians on both sides of the Atlantic have been weighing in, joining an increasingly vitriolic debate about the so-called Brexit's security and diplomatic consequences for Britain, Europe and America.

In recent years the European Union has been developing a common foreign and security policy which shapes the positions of the individual European members of NATO. Joint action has included agreeing on a common response to Iran’s nuclear ambitions and moving with sanctions on Russia after Moscow’s land-grab of Ukraine’s Crimea.

Weakening Britain's standing

U.S. President Barack Obama and British Prime Mini
U.S. President Barack Obama and British Prime Minister David Cameron hold a news conference following their meeting at 10 Downing Street in London, Britain, April 22, 2016.

During an April visit to London, U.S. President Barack Obama noted diplomatically in a newspaper opinion piece, “The United States sees how your powerful voice in Europe ensures that Europe takes a strong stance in the world, and keeps the European Union open, outward looking, and closely linked to its allies on the other side of the Atlantic. So the United States and the world need your outsized influence to continue, including within Europe.”

In later remarks, Obama was more blunt, talking less about Britain’s “powerful voice” and saying Britain would be at the back of the queue when it comes to a free trade pact with the European Union at the front. He said EU membership enhances Britain’s security and diplomacy, some of his aides are even franker, arguing an EU exit would seriously weaken Britain’s standing, making it less relevant and harming the trans-Atlantic security link.

One of Britain’s important post-World War II roles for Washington has been to serve as America’s deputy within the European camp, cajoling and lobbying on behalf of the United States, which was seen most dramatically before the Iraq invasion when then-British prime minister Tony Blair backed George W. Bush and rallied European doubters, except the French. With the European Union developing a common security and foreign policy, Britain wouldn’t be able to help influence it, reducing its importance to Washington.

FILE - A Pro-Brexit campaigner hands out leaflets
A Pro-Brexit campaigner hands out leaflets at Liverpool Street station in London, March 23, 2016.

Those favoring a British exit say the EU common security and foreign policy hasn’t worked out well. London University academics David Martin and M.L.R. Smith argue, “The European Union was founded in 1993, and from the Bosnia crisis at its inception, to its most recent attempts to sanction Vladimir Putin, the Russian president, and address the Syrian refugee crisis, through to the buying-off of Turkish autocracy, EU policy has looked a bizarre mixture of incoherence and appeasement.”

Maybe so, say proponents of continued EU membership, but they argue it would be a lot worse without British participation.

National security

This week, three of Britain’s former intelligence chiefs weighed in on the EU debate. Jonathan Evans, a former boss of the country’s internal security branch MI5, and John Sawers, former head of MI6, the external security service, wrote in the Sunday Times newspaper the European Union matters to security. They said by reducing intelligence data sharing with European partners Brexit could undermine Britain’s ability to protect itself.

Eliza Manningham-Buller, another former MI5 chief, entered the debate to warn that Britain “would lose influence.” Europe would be the loser, too, she said, when it comes to intelligence and information-sharing. Adding, by being inside the European Union Britain benefits from being able to raise the standards of some of the weaker European intelligence agencies.

Britain's Prime Minister David Cameron delivers a
Britain's Prime Minister David Cameron delivers a speech on the European Union at the British Museum in central London, May 9, 2016. Raising the stakes in Britain's European Union membership debate, Prime Minister David Cameron said Monday that leaving the bloc would increase the risk of war in Europe.

'Five Eyes'

But other former British spy chiefs remain less convinced about EU membership.

Richard Dearlove, who oversaw MI6 from 1999 to 2004, said, “the truth about Brexit from a national security perspective is that the cost to Britain would be low.” He, like other Brexiters, said the important intelligence relationship is with the so-called “Five Eyes” group, linking Britain, America, Canada, Australia and New Zealand, and that Britain provides more intelligence to Europe than it receives.

But the other Five Eyes members want Britain to stay in the European Union, because it makes it easier for them to co-operate via Britain with European intelligence agencies, both in practical and legal ways.

Manningham-Buller accused Dearlove of being out of touch with developments since leaving MI6 more than a decade ago. She said intelligence cooperation has improved dramatically between European spy agencies, having been forced to do so because of the rising jihadist threat, and Britain has played an important role in that development.