The English, normally skeptical about politics, have grown gullible. Both sides pelt the voters with forecasts of doom should the other side win. None are reliable, and the Leave figures have been especially deceitful. Remainers predict an economic armageddon of lost growth, a devalued pound and withered City of London. The Leavers’ Conservative leaders, assuming the mantle of a government in waiting, promise that “their” Britain could cover all the lost European subsidies and grants to farmers, poor regions, universities and schools. Evidence that they could find these additional billions is scant.

But there are deeper motives here than anxiety about the exchange rate or banks in London decamping to Frankfurt. Behind Brexit stalks the ghost of imperial exception, the feeling that Great Britain can never be just another nation to be outvoted by France or Slovakia. There’s still a providential feeling about Shakespeare’s “sceptred isle” as “this fortress built by Nature.” Or as an old Royal Marines veteran said to me, “God dug the bloody Channel for us, so why do we keep trying to fill it in?”

But in a Britain after Brexit, there will be internal border issues to worry about. London politicians look nervously north toward Scotland. Home to less than 10 percent of Britain’s population, Scotland has enjoyed a high degree of self-government since 1999. The pro-independence Scottish National Party dominates the country’s politics, consolidating its grip after losing a close-fought independence referendum in 2014.

Most Scots insist that they want to stay in the European Union. So what happens if a British majority says Leave and Scotland is dragged out of Europe against its will?

Many nationalists will demand an immediate new independence referendum. But Nicola Sturgeon, Scotland’s shrewd and popular first minister, will want to wait until polls show a settled majority of Scottish voters in favor of leaving the British state. It’s Ms. Sturgeon’s gamble that an economic downturn following Brexit, combined with the loss of European Union guarantees for workers’ rights and European subsidies for Scotland’s farmers and infrastructure projects, will deliver that support soon enough.

If Ms. Sturgeon’s strategy works out, Brexit could hasten the breakup of Britain. The constitutional fallout extends to Northern Ireland. A Leave vote would turn the open border between Northern Ireland and the Republic into a guarded frontier with Europe, since Ireland would remain a member in the union. This would undermine a major provision of the 1998 Good Friday Agreement, the peace deal that ended three decades of the Troubles.

Her Britannic Majesty would then be left with a simmering Ulster, the potential for resurgent nationalism in Wales, and a dominant population of 54 million English people. There is a logic to that, for Brexit is overwhelmingly an English, not a British, idea.