In September 1946, surveying the ruins of a continent ripped apart by war, Winston Churchill could see only one way to rebuild and preserve peace.

"We must build a kind of United States of Europe," Britain's victorious war time leader told an audience at Zurich University.

On December 31, 2020 under Prime Minister Boris Johnson -- who once wrote a biography of Churchill -- Britain finally left the European Union's single market

Another 27 countries remain inside the EU, and several more are keen to join, but Britain has always been an awkward fit. Does its island history explain why?

Until around 6500 BCE, the rocky promontory that would become Great Britain was attached to the coast of mainland Europe by the low-lying wilderness of "Doggerland".

As the Ice Age glaciers retreated, the hills and swamps were flooded and Doggerland became the bed of what is now the North Sea, and Britain stood alone.

After almost five decades in the community that has become the European Union, the United Kingdom left its institutions on January 31
After almost five decades in the community that has become the European Union, the United Kingdom left its institutions on January 31 AFP / Kenzo TRIBOUILLARD

The Mesolithic humans who had once hunted and gathered there were now pushed back to either shore.

British and European history would remain closely entangled, but also now physically divided.

Celtic tribes from the mainland displaced or mixed with pre-historic Britons and some, like the Belgae and the Dumnonii, lived on both sides of the Channel.

Invasion by the Roman Empire bound the archipelago into the Classical world until the fifth century, then -- as the legions left -- Angles, Saxons, Jutes and Scots arrived.

Various generations of Norse and Danish vikings raided and settled, until the Norman invasion of 1066 brought most of the island under a French-speaking monarchy.

With continental rulers, the influence of the Catholic church was consolidated, and for a while the kings of England held vast tracts of land in what would become modern France.

'We must build a kind of United States of Europe,' said Winston Churchill in 1946
'We must build a kind of United States of Europe,' said Winston Churchill in 1946 AFP / Justin TALLIS

Under Henry VIII, England left the Roman church, putting the island on a collision course with the Catholic crowned heads of Europe.

In 1558, the forces of Mary Tudor, Queen of England and Ireland, lost control of the port of Calais to the French, and Britain's history began to diverge from that of the mainland.

Elizabeth I defeated a Spanish invasion fleet in 1588, and her kingdom turned its eyes to the sea, becoming a maritime power rather than continental one.

French historian Francois-Charles Mougel sees this as a "180-degree" pivot that would set Great Britain apart from its neighbours, perhaps for good.

Ties with the mainland were not wholly cut. The British royals intermarried with the great houses of Europe, trade often flourished and wars often erupted.

But by the 19th century -- after the British Empire had allied with smaller continental powers to defeat Napoleonic France -- London ruled a global domain.

A map featuring the European Union without Britain
A map featuring the European Union without Britain AFP / Thierry Zoccolan

In the first half of the 20th century Britain would again become embroiled in fighting on mainland Europe in two world wars.

Britain, while it had been bombed, had escaped invasion and avoided both Nazi tyranny and a Communist post-war takeover -- reinforcing its sense of splendid isolation.

"There was a brief period after the Second World War when the Brits genuinely believed that we were at the height of our powers," said Anand Menon, professor of European Politics and Foreign Affairs at King's College London.

Along with Britain's dominions and colonies there was an alternative ally, as Churchill's French wartime rival and comrade General Charles de Gaulle knew all too well: the United States.

With the Empire starting to break up, Britain fell back as much on a "special relationship" with Washington than it did on Europe.

France was also a shrinking empire in search of a role, and it thought it had found one in the emerging European Community.

In 1963 and 1967, De Gaulle vetoed British requests to join the bloc, fearing a loss of influence for Paris to Britain, with its close ties to the US.

Britons would have to wait until 1973, and the French presidency of George Pompidou, to join what was to become known later as the European Union.

"One of the things the French made sure the European Union did before we joined was rigging its budgetary arrangements so they didn't work for us," Menon, who founded the think tank Britain in a Changing Europe, told AFP.

"Part of that was to lock in a system with a view to the Brits joining, and within 18 months of us joining we were complaining about the budget rules."

But Britain's sense of distance did not end there. Although as an up-and-coming Conservative politician, Margaret Thatcher had been a convinced European, she soon clashed with Brussels.

While the core European countries were pushing for deeper political and legal union, Britain was pushing for a wider, looser common market.

London championed the arrival of former Soviet-bloc members keen to taste the fruits of capitalism, and resisted regulatory integration.

Even the 1994 opening of the Channel Tunnel did not close the divide.

Thatcher was succeeded by the relatively europhile John Major, and he by Labour's Tony Blair, who pledged to govern from the "heart of Europe".

But London kept and expanded the opt outs that Thatcher had negotiated from core EU rules.

Britain received a rebate on its dues, never joined the Schengen passport-free zone and most famously did not join the union's most ambitious project: the euro.

Then in 2016, faced with a tide of eurosceptic populism from his right flank, Prime Minister David Cameron tried to heal the Dogger Divide once and for all with a referendum on EU membership.

And, once again, Britain pivoted away from the continent.