Poised with her pen in front of an outsize Union flag and a No 10 portrait of that old rogue Robert Walpole, Theresa May got all the headlines she’d hoped for when she signed her Article 50 letter—headlines about a “momentous day in British history.” In truth, the state that’s been there since the days of the Whig wheeler-dealer behind her is coming unstuck, and we may be approaching the end of days where “British history” is concerned. What’s more, her signature will only encourage that.

The first 250 years of the Union with Scotland were for the most part a confident affair. Union with Ireland was more brutal, even before the violent rupture of the 1920s. Even so, in the aftermath of the Second World War—a war always described as won by Britain, and not its components—the United Kingdom that combined the Northern Irish residual with England, Scotland and Wales was not merely a strong nation state, it was deemed the very model of stable, unified state. As Britain lost an empire, it notoriously struggled to find a role, but at least it didn’t worry about its identity: this was a country that knew who it was.

Looking back, however, one can now trace an incremental tendency for things to fray over half a century. It was in the late 1960s the Northern Irish troubles began, and the Scottish nationalists began to break through. But as decolonisation, devolution, power-sharing and waves of nationalist sentiment came and in some cases went, from 1973 onwards everything unfolded within a single European club with common ground rules. Perhaps that encouraged a very British belief that it would be possible to muddle through. No longer. Britain—or rather the English and Welsh parts of it—has resolved to leave the club, and is fast discovering it could lose its unity too.

In this issue, Neal Ascherson, a Scottish independence enthusiast, puts Britishness under the microscope, and finds its chief component is an English nationalism which assumes unhealthy forms precisely because it is rarely openly expressed, while John Curtice highlights the finely balanced psephology on which the fate of the Union hangs. As an economist, Frances Cairncross starts from the premise—which many Prospect readers will share—that prosperity will be diminished rather than enhanced by creating new borders on these islands, but she concludes that the price of separation may not be severe. Simon Jenkins concurs, arguing that all that matters is agreeing sensible cross-border arrangements to regulate trade.

Such arrangements, however, may be hard to achieve between one state that’s in the EU and another that’s outside. In both halves of Ireland, there is—as Denis MacShane writes—rising alarm that a hard Brexit will mean a hard border. Gibraltar, a further-flung outpost of the Union Jack, is beset with anxiety. That may or may not be soothed by former Conservative leader Michael Howard’s apparently earnest suggestion that Britain stood ready to turn the navy on Spain on the rock’s behalf.

What an irony it would be if the upshot of leaving Europe was to shunt the BR in Brexit to the same siding of history as British Rail. It is, however, an irony that it is becoming ever easier to envisage coming to pass.