Finally, after 1,317 days of confusion, rancor and endless votes in Parliament, it was time for Brexit. On Wednesday, teary members of the European Parliament joined hands and sang “Auld Lang Syne,” a clock projected onto 10 Downing Street counted down the minutes, Union Jacks lined the Mall before Buckingham Palace.

“A new dawn for Europe,” leaders of the European Union wrote in a joint article.

“This is the moment when the dawn breaks,” came Prime Minister Boris Johnson’s echo from across the newly raised dividing line.

It was left to the dispassionate BBC to cut short the bathos: “Brexit is far from ‘done,’” the Beeb coldly said, before listing the many travails still to come, most notably the negotiations that now begin with the E.U. on the details of Britain’s future relationship with the Continent. Mr. Johnson has promised not to seek an extension of the Dec. 31 deadline, which he could, though a less complex trade deal between the E.U. and Canada took seven years to finish. To discourage other members from exiting, the E.U. is not likely to cut Britain much slack on E.U. standards and rules in their trade.

Britain will now also reach across the Atlantic to what President Trump has held out as a “very big trade deal, bigger than we’ve ever had with the U.K.,” which Mr. Johnson has touted as a benefit of quitting the E.U. That, too, could prove a disappointment. A former British ambassador to Washington, Kim Darroch — who resigned in July after his derogatory comments about Mr. Trump leaked out — was among those who noted in interviews that Mr. Trump is not given to generous trade concessions, least of all in an election year.