Of all the agonizing twists and turns of British politics since the narrow vote on June 23, 2016, in favor of leaving the European Union, the landslide victory of Boris Johnson in Thursday’s election may be the most stunning. It also should have been the most predictable. After three and a half years of debilitating debate, the British had simply had enough and rallied to Mr. Johnson’s unvarnished slogan, “Get Brexit Done.” They are likely to get a very different Britain in the bargain.

The scale of the Conservative victory, and the extent of its conquests in traditional Labour strongholds, may not have been anticipated. But the bullish reaction from the markets, and, somewhat more counterintuitively, the celebration in Brussels, were further reflections of a widespread sense that “If it were done when ’tis done, then ’twere well it were done quickly.”

Brexit is now a fact, and that is the first and most concrete takeaway from the election. Without any viable opposition in his own Tory ranks, whose dissidents he had purged before the election and whose deputies all vowed to support him on Brexit, Mr. Johnson is likely to get his Brexit bill through Parliament within days or weeks, and Britain to formally leave the union by the end-of-January deadline. For many Britons and for many leaders of the Continent, ending the endless bickering and the threat of a chaotic deal-less Brexit was a source of great relief.

But that does not mean Brexit is “done,” or that it will be done quickly. It only opens the next, and arguably more difficult, stage of disentangling Britain from the vast and complex economic relationships that form the customs links and single market of the European Union, a task that is supposed to be done within a year but probably won’t be. Throughout the campaign, in which Mr. Johnson was successfully shielded from too much exposure to too many difficult questions, he gave little sense of how he would proceed.