BRUSSELS (AP) — The New Year could finally bring a fresh start and a commitment to let bygones be bygones for Britain and the European Union.

But don’t bet on it.

The U.K. has chosen to leave the EU, setting a course away from the continental mainland. But the two sides’ histories have been too intertwined for 1,000 years for the split to be simple.

Eleven months after Britain’s formal departure from the EU, Brexit becomes a fact of daily life on Friday, once a transition period ends and the U.K. fully leaves the world’s most powerful trading bloc.

But customs controls, red tape and the residue of bile caused by years of acrimonious divorce talks may provide the sting — not the thrill — of the new. And despite the 4½-year extraction process, loose ends will surface for months, even years, to come.

“For one reason or another, the U.K. is likely to be in non-stop negotiations with the EU for decade after decade,” said Charles Grant of the Center for European Reform think-tank.

Brexit marks the end of an awkward relationship. Britain joined the then-European Economic Community in 1973, but never fully embraced the bloc's project of ever-closer integration. The EU was born out of the ashes of World War II and its delusional, destructive nationalism.

As a nation victorious in two world wars and with lingering memories of its imperial past, Britain viewed the pan-European project much differently than, for example, Germany.

Still, actually leaving the bloc was long a fringe idea before it gradually gained strength within Britain's Conservative Party. In a 2016 referendum, voters — striking a blow against the status quo — opted by a narrow 52% to 48% to leave.

That political shock is still reverberating. It took 3½ years for the split to happen last Jan. 31 and another 11 months of fractious wrangling to agree on a trade deal.

Sealed on Christmas Eve, it satisfies major demands on both sides. It protects the EU market by making tariff-free trade conditional on Britain continuing to meet high social, workplace and environmental standards. It allows Britain to claim it regained “sovereignty” because it is no longer part of EU structures like the European Court of Justice.

U.K. Prime Minister Boris Johnson called it “a new starting point for our relationship between sovereign equals.”

The EU begs to differ. It considers itself the superior partner, a bloc of 450 million consumers, carried by economic juggernauts like Germany and France, while Britain is a nation of 67 million. It believes the brunt of Brexit's pain will be borne by the U.K.

“We are one of the giants,” said European Commission President Ursula von der Leyen. “From a position of strength you can achieve a lot.”

For British Brexiteers, the blow from erecting new trade barriers is compensated by visions of regained freedom and — a crucial word — “sovereignty.”

So far, Johnson has given only glimpses of what he plans to do with that sovereignty: forge trade deals around the world, be more competitive through “smarter and better regulation,” expand the high-tech sector.

From a member of the EU, Britain will become its economic rival.

The EU is wary of Britain's potential to seek an edge by slashing standards and becoming a low-tax “Singapore on Thames” on its doorstep. That’s why the Brexit deal contains “level playing field” restrictions on how far the U.K. can diverge without punishment.

Britain may soon bristle against those restraints. Further tensions, spats and negotiations loom in the future. And the agreement is to be reviewed every five years, reviving a debate that many had hoped would fade.

“If you thought this was going to be a one-hit wonder … then you will be disappointed,” said Catherine Barnard, professor of EU law at Cambridge University.

Trust is already in short supply, and recent events gave a taste of what can happen.

Much U.K.-EU goods trade goes across the English Channel, and when France decided to stop all crossings on Dec. 20 in response to a fast-spreading new COVID-19 variant identified in England, it created traffic chaos that took days to unsnarl, even after Paris lifted the blockade.

Britain’s tabloid press perceived French ill-intent, accusing Paris of trying to force Britain into a Brexit trade deal. French President Emmanuel Macron became “Monsieur Roadblock” giving Britain, in keeping with the Christmas theme, a “kick in the baubles.”

France denied the border closure was related to Brexit. But Macron’s office issued a statement this week saying that “France will be very vigilant from Day 1" in case Britain "disregards its commitments” under the agreement.

While Johnson sets Britain out on a solo course, most EU leaders see cooperation as ever more vital, especially with a pandemic ravaging the world and the United States and China ready to squeeze European nations out of the diplomatic game.

”We should cut through the soundbites and ask ourselves what sovereignty actually means,” said von der Leyen. “It is about pooling our strength and speaking together in a world full of great powers. And in a time of crisis, it is about pulling each other up instead of trying to get back to your feet alone.”

Brexit has already destabilized the U.K. itself. It has boosted support for independence in Scotland, which voted strongly to remain in the EU in 2016. Northern Ireland, which shares a border with EU member Ireland, remains economically closer to the bloc than the rest of the U.K. under the departure terms, a status that could push it away from Britain.

EU chief Brexit negotiator Michel Barnier, a European Unionist and Frenchman to the core, sees tough times ahead for Britain.

Barnier said that “when you see today’s world, a dangerous, unstable and unjust world, I definitely think that it is better to be together, with our neighbors in a union, a single market, than everyone being in their own corner, with their own interests.”

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Lawless reported from London.

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