Things could not have gone better for British Prime Minister Boris Johnson. His ruling Conservative Party scored a thumping victory in last Thursday’s elections, securing dozens of new seats to forge a solid majority in Parliament. Johnson won what he long wanted: a mandate to follow through on Brexit, the siren song that drove his quest to power.

Now his opposition is fractured and plunged in new rounds of recrimination and squabbling. The Remain movement, which had pinned its hopes on a hung Parliament forcing a second referendum that could halt Britain’s slide out of the European Union, was crushed.

Johnson, the blustering former London mayor and Tory gadfly, gambled when he threw his lot behind the Leave campaign ahead of the 2016 referendum. He hitched his political fortunes to the Brexit project, peddling rosy visions of a post-imperial Britain unshackled from Europe. He grandstanded over national sovereignty and demonised immigrants.

There were times when it seemed he made a bad bet. Political paralysis at home and the mess of splitting from Europe led him to resign as foreign secretary in former Prime Minister Theresa May’s government. After May’s unhappy time in office came to an end, Johnson won an internal Conservative Party vote to take her place.

His zeal for Brexit — he made getting it “done” his catchphrase — tilted a tough electoral landscape decisively in his favour. He was aided, too, by the apparent unsuitability of Jeremy Corbyn, the left-wing Labour leader who was historically unpopular, even though many of the policies he pushed were not. On the promise of Brexit, whole swaths of once traditional Labour seats in the Midlands and North of England abandoned Corbyn for Johnson.

“Johnson could win only with Brexit, changing voter demographics, and the good fortune of running against Corbyn, the most unpopular candidate for prime minister in modern history,” wrote The Atlantic’s Tom McTague. “Had Johnson strayed off course, he would have sailed into the strong countervailing winds working in Corbyn’s favour: a decade of austerity, pitiful earnings growth, increased waiting times at hospitals, and the desire for something new.”

But Johnson still has his work cut out. On a post-election victory lap across various parts of the country, he promised “healing” and efforts to bring unity. After all, though he won the bulk of the seats in Parliament, Johnson’s Conservatives still commanded less than 45 per cent of the actual vote. More years of Tory austerity may see the voters who flocked to his banner this time not return the next round.

In a parting salvo widely pilloried for a lack of humility, Corbyn suggested that Johnson’s mandate is weaker than he thinks. “I am proud that on austerity, on corporate power, on inequality and on the climate emergency we have won the arguments and rewritten the terms of political debate,” Corbyn wrote in an op-ed in The Guardian. “But I regret that we did not succeed in converting that into a parliamentary majority for change.”

Moreover, as The Washington Post reported on Saturday, unity may be a tough pitch. While the Conservatives dominated in England, the victories of pro-EU nationalists in Scotland and Northern Ireland suggest troubles ahead. The Scottish National Party explicitly believes it has the mandate for a second independence referendum, even though it’s unlikely that a Westminster in Johnson’s thrall will permit it. And, once Brexit is carried out, Northern Ireland may find itself politically cut adrift from the rest of Britain.

“Boris has won his gamble in England definitely and also in Wales, but the price is that you exacerbate divisions and you create a state crisis,” Richard Wyn Jones, a politics expert at Cardiff University, told my colleagues Karla Adam and William Booth. “The whole future, the territorial integrity of the state, is clearly in question in Scotland in Northern Ireland.”

Then there’s the actual process of really implementing Brexit. With Parliament on his side, Johnson will probably at the end of next month initiate a process that will place Britain in an 11-month transitional phase, during which it will negotiate a possible free trade deal with Europe and further terms of the divorce.

Analysts consider that an absurdly narrow window through which to settle a pact of this complexity. They point to Canada’s own free trade deal with the European Union, which took seven years to negotiate and still isn’t fully ratified. Britain’s trade with the rest of Europe is exponentially larger than that of Canada.

“Virtually no one on the European side thinks it is possible to agree to a meaningful trade deal that quickly — meaning either Johnson will have to break a campaign promise by asking to extend the transition, or Britain and the EU could wind up with the same type of sudden break at the end of 2020 that Parliament until now refused to allow,” reported my colleagues Michael Birnbaum and William Booth.

Some right-wing British proponents of Brexit have imagined a post-EU future where Britain can become a “Singapore-upon-the-Thames”, a business-friendly haven of low taxes and fewer bureaucratic barriers to entry. That’s not an approach many in Europe will welcome.

“My message to the British is that the more loyal we are to each other, the more our relationship will be close,” said French President Emmanuel Macron over the weekend. “But don’t think you can have an extensive trading relationship, a maximal access to the European market, with substantial differences on sanitary, climate, economic and social regulations. This is not true.”

• Ishaan Tharoor writes about foreign affairs for The Washington Post. He previously was a senior editor and correspondent at Time magazine, based first in Hong Kong and later in New York