How do some countries tear themselves apart while others keep together? To the most vital question facing the UK the answer, it seems to me, lies in an old book. I can picture its burgundy spine, the black-and-white cover photo of three solemn Asian gentlemen wearing tuxedo jackets and sarongs, and that title so good it is almost a spell: Imagined Communities.

Perhaps you know this classic study, for its influence still reverberates nearly 40 years after its first edition. In it, Benedict Anderson analyses how a national identity is manufactured, so that a hunk of rock can inspire men and women to kill or die for it. Prescribed it by a university teacher a quarter-century ago, I felt the thing practically buzzing with timeliness. The Soviet imperial glacier had broken apart, and politicians and pundits assumed the world’s future lay in the nation state. Yet Anderson argued that what nationalists saw as ageless and undeniable was to scholars modern and confected. The nation, he wrote, “is an imagined political community” – a collective waking dream fed by comparatively recent technology, such as books in the vernacular language and common consumption of the same media. Reading the same newspapers as one’s neighbours on the train or in the barbers reassured citizens that their “imagined world is visibly rooted in everyday life”.

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Every nation state stands on the brink of death; keeping it alive requires constant reaffirmation. And while the late Anderson focused on Asia, his argument applied equally to the land of warm beer and old maids cycling to communion. In her history, Britons, Linda Colley nailed Great Britain as “an invented nation superimposed, if only for a while, on to much older alignments and loyalties”.

If only for a while. That clock has been ticking, and the time is nearly up. What was proclaimed in 1707 the “one united kingdom by the name of Great Britain” is slowly crumbling into pieces before our eyes. Chip! This week it is Nicola Sturgeon promising Scots another independence referendum by next Christmas. Chip! Last week it was a Telegraph column asking whether Britain really needed Northern Ireland.

The same bunch of spivs, blimps and yahoos chuntering on this week about making Britain great again are more likely to shrink it, making the country smaller than at any time since the Stuart dynasty. Weeks before the 2016 referendum, the then prime minister David Cameron warned that Brexit meant national “disintegration”. This summer his predecessor, Gordon Brown, said: “The union is today more at risk than at any time in 300 years.” When it finally comes, the proximate cause of the UK’s demise will be easily ascribed: it will be death by hard Brexit. However deep the warm bath of mutual congratulation sunk into by commentators and backbenchers at potentially seeing off a no-deal Brexit, it doesn’t hide the ugliest truths. Westminster is still heading out of Europe, and Boris Johnson’s proposed deal is harder and worse than the one punted by Theresa May. Failing to agree on anything apart from the disaster of crashing out of the EU, the rebel alliance has won a battle yet stands to be utterly routed in the war.

All that’s left now is the most negative way to hold together a nation: an imagined enemy

If Brown shares just one thing with Labour’s current leadership, it is this: both calculate that the Scottish nationalists will win a landslide at the next general election, spurring Sturgeon to demand a new referendum that Westminster will find near-impossible to deny her. And if remain-voting Scotland goes its own way, it will make the Brexit maelstrom look like a passing shower. Westminster’s relationship with the EU goes back four decades. The union with Scotland stretches back three centuries. At Stormont, the political arithmetic is far harder, but politicians there must be taking legal advice on whether the framework of the Good Friday agreement allows Northern Ireland to leave the UK and rejoin the EU.

Signs of this were visible even in the hours after the 2016 vote, but the recklessness with which the Tory party has behaved since has made it only more likely. Look no further than Johnson, who spent decades at the Telegraph sneering at the “Jocks”, their hands outstretched for pocket money from “Uncle Sugar” down in Westminster. As London mayor, he declared: “A pound spent in Croydon is of far more value to the country than a pound spent in Strathclyde.”

No wonder that when Scottish Conservatives battled this summer to block Johnson from becoming party leader they adopted the codename of Operation Arse. Yet Johnson’s greatest gift is to take the conventional wisdom of his class and wrap it in outrage, and on Scotland he is no different, merely expressing the attitude of a ruling party that long ago stopped giving a tinker’s cuss about anywhere or anything too far over the horizon.

While it would be easy to argue that this national instability is the fault solely of Johnson and his red-faced, jabby-fingered Brexiters, it would also be wrong. What just a few weeks ago Brown bemoaned as “the unravelling of a community of mutual interests, common purpose and shared ideals” is a process that has taken decades and that has been led by London.

As described by Anderson and others, nation-building is a long journey of establishing common interests and promulgating them through an engineered common culture. The UK’s store of both is dwindling. As recently as Margaret Thatcher, the country had only four TV channels; today you need never watch the same thing as either your spouse or your offspring. Last year, media watchdog Ofcom reported that the third-biggest source of news in the UK was Facebook. It also found that social media users were mainly drawn to stories that were “trending”, or “commented on by friends or people that I know”.

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So a national culture gets fragmented into friend groups, even while the idea of a national economy becomes ever more laughable. Thatcher’s legacy is summed up by historian David Edgerton in The Rise and Fall of the British Nation as “a new kind of economy … in which there was no longer any concern for national champions, for the ownership even of firms operating in the UK, or the balance of payments”. What’s left is the idea of a territory sullenly governed and subsidised by central London, which is itself a global entity – a host to multinational businesses, an international tax haven, a laundromat for the world’s hot and dirty money.

What happened to the idea of Britain? It lost the interest of elites who spotted easier, quicker ways to cash in. Perhaps all that’s left now is the most negative way to hold together a nation: an imagined enemy. It used to be the Frogs and the Krauts and all those other ugly words. Now? It’s the SNP, who used to have Ed Miliband in their pocket, the saboteurs in the judiciary and in parliament, the remoaner establishment. The inventiveness essential to hold a nation-state together has largely disappeared, to be supplanted by the language of loss and sourness and existential threat.

• Aditya Chakrabortty is a Guardian columnist