See more of the story

It was November 2014. An election loomed and David Cameron was in trouble. The polls put the British prime minister's Conservatives on course to lose a knife-edge vote to the Labour Party.

Then, his team hit on a plan that changed everything. At its heart was the idea the Tories could win if they focused on appealing to voters in England, effectively giving up the fight north of the border in Scotland.

It worked, and Cameron won, but the full impact of that campaign four years ago could prove to be even more seismic than the vote to leave the European Union which followed his victory. After another Tory election triumph, with nationalists ascendant in Scotland as well as Northern Ireland, the very existence of the U.K. itself may be at risk.

The Conservatives have won the U.K.'s past four elections but on only two of these occasions have they secured an outright majority in Parliament — in 2015 under Cameron, and again on Dec. 12 this year with Boris Johnson.

First Cameron and then Johnson won big by convincing voters across England to back the Tories. Both times they warned that a weak Labour leader would be pushed around in a coalition with the Scottish National Party. And both times, the Tories lost territory in Scotland, while the SNP surged to its own landslide.

The pattern is set. To win a majority, the Tories have largely abandoned Scotland and demonized its leaders. The result is that Britain is deeply split between these two ancient nations.

Halfway from London to Scotland lies Wakefield. In a former mining area of northern England, the city had been held by Labour since 1932 until Johnson's Tories swept through.

At 4 p.m. on the last Sunday before Christmas, dusk is falling outside the cathedral.

People have come to pray on this site for more than 1,000 years. It's 10 days since Johnson's emphatic victory, which he won by convincing traditional Labour voters in areas like this to back his Tories and "get Brexit done."

The hatred of the Tories had been entrenched. Wakefield was one of the first areas to suffer the closure of coal mines under Margaret Thatcher's government in the 1980s. Despite the history, Johnson's campaign resonated among the 66% of the city's voters who backed Brexit in 2016, and they chose a Tory MP.

While settling the E.U. question was crucial for the election, Johnson also needed to show voters who their enemies were. According to his campaign, these foes were members of Parliament who had spent three years thwarting Brexit, including many Labour politicians.

The enemy was also Scotland's first minister, Nicola Sturgeon, who would help Labour leader Jeremy Corbyn claw his way into power, and call the shots in a shaky coalition. Johnson urged voters to deliver a Conservative majority instead.

According to John Denham, a former Cabinet minister in Gordon Brown's government, the left must take a portion of blame for Johnson's success. Corbyn, like Ed Miliband in 2015, failed to speak for English voters who feel sidelined by the social and economic changes of the past 30 years.

"The line that runs through these two elections in 2015 and 2019, and the Brexit referendum, is the sense that these English voters did not have a voice," says Denham, director of Southampton University's Centre for English Identity and Politics.

The 2019 election has widened the political divide between England and Scotland.

The politics of English and Scottish identity looks set to play into the future battles, pulling the two countries further apart. Sturgeon has already stepped up demands for a fresh vote on independence, as England takes Scotland out of the E.U. next month.

Some believe the only chance to save the U.K. is to move to a federal model for its constituent nations. Others say Scottish independence will be inevitable after Brexit.

If Cameron had not chosen four years ago to exploit the antipathy of English voters toward Scottish nationalists, the U.K. may have looked very different today.

Yet it worked out well for one man, quick to spot the genius in that tactic at the time.

"What was brilliant about the campaign was spotting this phenomenon of the SNP surge and then turning that into the story and creating this incredible narrative," Boris Johnson said a few months after Cameron won. "It was a brilliant tactic. Absolutely brilliant."