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Britain's outgoing Prime Minister, Boris Johnson, leaves 10 Downing Street to attend Prime Minister's Questions in the House of Commons on July 13 in London.

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John Rapley is a professor at the Johannesburg Institute for Advanced Study. His most recent book is Twilight of the Money Gods.

Ever since he was a little boy, when he dreamed of being “world king,” Boris Johnson wanted to leave his mark on history. In the end, he got his wish. Unfortunately, he may not like what the historians will say about him.

In the political obituaries being written for the fallen British Prime Minister, it’s hard to find much praise. The one thing which redeems him in the right-wing press, that he “got Brexit done,” is the one thing they will probably rue with time. That’s because Mr. Johnson’s entire approach was to deliver the goodies on a layaway plan. He will now pass that bill to his successor. It’s a steep one.

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With good leadership, the next prime minister could repair some of the considerable damage Mr. Johnson did to the country’s system of government. Although it always seemed a stretch that he was merely “Britain Trump,” as the admiring former president once called him, in the end his narcissism did reach Trumpian proportions. In the final hours of his fight for power, he insisted he’d never resign because the people had selected him personally, a claim for which there is no basis in Britain’s parliamentary system. He then concocted various dubious schemes in a desperate bid to hold on. His MPs, watching anxiously as voters hammered the Conservatives in one by-election after another, would have none of it. Unlike Donald Trump, Mr. Johnson found his dwindling constituency valued its constitution more than it did him. Britain, it turns out, likes its democracy.

Just as statesmanlike behaviour could restore the “good chap” principles on which Britain’s unwritten constitution has long rested, so could it repair some of Britain’s reputation on the world stage. Although Mr. Johnson got credit for his strong stand on Ukraine, elsewhere, his penchant for petty squabbling with the Europeans, and for signing and then immediately breaking treaties, degraded Britain’s once-prized international standing. A new leader could send a strong message that “Britain is back.”

Mind you, there’s no guarantee of that. Britain’s Conservative Party is now stuck in a ditch it could struggle to escape. To neutralize threats to his authority, Mr. Johnson purged the party of much of its best talent. As a result, a leader chosen from a less-than-stellar pool of candidates will take over a party that, for all its storied history, faces a clouded future. And Mr. Johnson’s Brexit actually leaves them a poisoned chalice.

The economic data are clear and overwhelming: The British economy, which never really recovered from the 2008 financial crisis, was further walloped by Mr. Johnson’s rushed Brexit deal. The country’s break with its largest market has knocked a chunk out of the economy, punished exports and hobbled businesses. To make up all the lost revenues caused by the weak economy, the government has had to raise the tax burden to its highest level since the 1950s. Even then, the worsening public finances have resulted in continued erosion of public services. As hospital wait times grow ever longer, offences such as rape have been all but decriminalized, so rare are the convictions that depleted police and court systems can deliver.

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The country knows this. Polls suggest that a majority of Britons now feel Brexit was a mistake. Were a referendum held today, most would probably vote to rejoin Europe. However, the small minority who still support Brexit overwhelmingly vote Conservative, making that diminishing pool of support the base the party must please.

As the state of the economy and public services worsened and voters left the party, Mr. Johnson reverted to a divisive political strategy of rewarding Conservative supporters and not only punishing everyone else, but mocking them for objecting. This has shored up the party base, but at the expense of alienating a large majority of voters. This spurned group now so loathe Mr. Johnson’s party that, as recent by-elections have shown, they have begun engaging in tactical voting to support whichever candidate will defeat the Tories. The risk of a future Conservative election wipe-out remains real.

To avoid this, the incoming prime minister will need to reboot the economy and rebuild the country’s public services. But for as long as Brexit remains a no-go zone to Conservative leaders, the fundamental wounds to Britain’s economy will go untended. The country’s per capita income, which on the eve of the 2016 Brexit vote stood level with Germany’s and Canada’s, has now fallen behind them by a tenth. Its outlook remains grim. The only economy in the Group of 20 with bleaker prospects is Russia (whose economy is in free fall), while Britain’s inflation is the worst in the Group of Seven. Not for nothing is it now called the sick man of Europe.

The day will come that the Brexit taboo is lifted and a new government finally begins to treat the economy’s illness. By then, though, the country may have fallen so far backward that it has no hope of ever making up the lost ground. At that time, historians will provide a full assessment of Mr. Johnson’s legacy.

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He may thus spend his dotage reading of how, after promising to restore Britain’s greatness, he left the country a tattered shell of its former self.

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