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Friday marks “Brexit Day,” the moment when Britain finally leaves the European Union after more than three years of painful negotiations and political squabbling.

So, get ready to pop the corks of your English sparkling wine — but don’t celebrate too much. As The Washington Post’s William Booth and Karla Adam point out, not much will change at first: The toughest work on Brexit is still to come, including matters as diverse as negotiating fishing rights and printing passports.

But perhaps the most difficult challenge ahead is one many Brexiteers had hoped would be the easiest: Britain’s post-Brexit relationship with the United States.

Britons have long cherished the “special relationship” between London and Washington, and many pro-Brexit voices, including campaigner Nigel Farage and Prime Minister Boris Johnson, have suggested that leaving the E.U. will help strengthen transatlantic ties with “Mr. Brexit” himself, President Trump.

The reality, however, may be trickier. When Britain leaves the E.U., one of its first tasks will be to negotiate a bilateral trade deal with the United States. Trump has promised a “massive” deal, but experts suggest that he will continue his hard-nosed, winner-takes-all approach.

“Trump is not going to be doing Johnson any favors,” Amanda Sloat, a senior fellow with the Brookings Institution in Washington, told Reuters this week. “He’s not going to give him a trade deal without major concessions.”

And it’s improbable that Johnson will offer any favors in return. This week, the British prime minister reminded the world that even after Brexit, Britain isn’t willing to simply fall in line with its American ally.

The British government announced on Tuesday that it would allow the Chinese tech giant Huawei’s equipment to be used in 5G telecommunications networks in the country, marking a major split with the United States, which had pushed allies to refuse to work with the company.

Huawei is the world’s leading provider of 5G equipment and is often able to offer its services at highly competitive prices, but the United States warns that it is too closely linked to the Chinese government, and Washington has called for a boycott of the company.

British Foreign Secretary Dominic Raab played down the decision, suggesting in a statement that Britain would “never take decisions that threaten our national security or the security of our Five Eyes partners,” a reference to intelligence sharing among Australia, Canada, New Zealand, Britain and the United States. Huawei was barred from the “core” of Britain’s mobile networks and strategically important sites, and its market share was capped at 35 percent.

But many Trump allies — and even some Trump foes who share his skepticism of Huawei — voiced their anger at the move. Some wonder how the impetuous American leader will react now that Johnson has resisted his pressure.

“Big call by Boris on Huawei, not least because it will infuriate President Trump,” Piers Morgan, the ever-controversial British TV pundit tweeted Tuesday. “This should end any fears our Prime Minister will be a lapdog to the White House.”

It was only a matter of time before the Trump-Johnson honeymoon came to an end. Though Trump openly supported Brexit and Johnson, and the British leader has aligned himself more closely with Trump than with any Western European leader, they are very different men.

Johnson is an Oxford-educated polyglot who has spent much of his life living abroad and hobnobbing with political elites. Trump is … well, you get the point. Poll after poll shows that Trump is deeply unpopular in Britain, a fact of which Johnson is probably cognizant.

Even those in the British establishment roughly aligned with Trump don’t always speak kindly of him. David Davis, the former Brexit secretary who supports the U.S. view of Huawei, told the BBC on Wednesday that Trump was “transactional” and “won’t remember what happened this week.”

But the differences between Britain and the United States may go beyond Trump. “Simply put, many Europeans, even many Britons, believe their political and economic models are superior to ours,” Henry Olsen wrote for The Post this week.

Few in Britain support China’s rise, but there are some who view it as a lesser evil. According to Pew Research Center data, fewer Britons have confidence in Trump on the world stage than in Chinese President Xi Jinping.

Trump may have seen himself in the rebellious and rambunctious Brexiteers, but the heart of that movement was thoroughly British. It was tied up with emotions about a loss of influence on the world stage and the creep of foreign influence into Britain. That doesn’t sit well with Trump’s “America First” philosophy.

Trump’s United States and Johnson’s Britain may end up on the opposite side of a variety of issues, from the Iran nuclear deal to the use of chemicals to wash American chicken. They may find some time to share in the celebration on Friday, but there could be a long hangover for Johnson — especially if Trump is reelected.