“The British always had one foot in the E.U. and one foot out — now with Brexit they want the opposite,” a French legislator, Jean-Louis Bourlanges, joked. But Britain had strong allies among the other member states. Its first-class diplomatic personnel brought welcome expertise to Brussels. Even the French understood that the British were a powerful addition to this extraordinary supranational entity.

Hence the idea, sold by the Brexiteers to their voters, that negotiating new terms with the Continent would be easy because the union had so much to lose in this divorce. It was even expected that other countries would follow Britain’s example. “Who’s next?” was the only question that President Trump put to the European Council president, Donald Tusk, in a phone call in January 2017, I was told by Anthony Gardner, the American ambassador to the European Union at the time.

It didn’t turn out that way.

The European Commission designated an affable Frenchman, Michel Barnier, as its chief negotiator and gave him a team of 60, drawn from 19 nationalities. A former center-right member of Parliament, government minister and union commissioner, Mr. Barnier viewed Brexit as “a lose-lose case” and was determined that the union would lose as little as possible. He understood early that unity would be crucial and repeatedly toured the remaining 27 member countries to brief their leaders.

Doing so, Mr. Barnier observed “a new feeling of gravity” among European leaders, spurred by the realization that faced with Donald Trump and Vladimir Putin, Europe must not be weakened by Brexit. So instead of intra-European infighting and rivalry, London’s negotiators met a solid wall of unity and sense of purpose. The European Union became so relevant that exiting it is not an issue anymore, even among the most Euroskeptic governments.

Some are more patient than others. Last month, President Dalia Grybauskaite of Lithuania tweeted her Christmas wish for Britain: “Finally decide what you really want and Santa will deliver.” On Twitter, Germany’s economy minister, Peter Altmaier, professed admiration for “the epic struggle of the British people,” and more than 20 German politicians and celebrities wrote Britons an emotional letter saying they “would always have friends in Germany and Europe.” Others secretly pray for Britain to finally leave, solve its domestic problems and come back in a decade or two.

The ball is in Britain’s court, facing a March 29 date to exit with or without a deal. E.U. governments are accelerating preparations to face a no-deal catastrophe.

There was another, less glorious lesson from the Brexit saga. The uprising that brought it in 2016 was not, it turns out, isolated.