LONDON — For travelers returning to London from distant places struggling for bare-bones survival, the portals of passport control sometimes resemble what a British writer likened to the looking glass in Lewis Carroll’s fable of Alice.
Out there — in the Middle East, Africa and elsewhere — wars rage, drought crisps the land, democracy strains against despotism.
And, in consequence of such upheaval, a procession unparalleled in modern times of war-zone refugees and other migrants seeking new homes in Europe challenges a political and social order that traces its roots to 17th-century notions of governance and sovereignty.
Here, though, it seems as if the sole prism through which issues are squeezed is a referendum on June 23 on British membership in the European Union — a yes-no decision thrown into even starker relief by President Obama, whose visit to London last week was centered on support for Prime Minister David Cameron’s commitment to remaining in the bloc.
In recent days, too — with huzzahs, gun salutes and the lighting of beacons to fete the 90th birthday of Queen Elizabeth II; celebrations in honor of the 400th anniversary of Shakespeare’s death; and commemorations of St. George, the patron saint of England — the debate has inspired fretful musings about the nature of Britain’s identity.
Whatever else, the ballot will bookend a long-running and visceral argument, confronting Britons with what many depict as an existential choice that has sown division beyond political factions to sunder friends and relatives.
Even the family of George Osborne, the pro-European chancellor of the Exchequer, has been reported to be squabbling over Brexit, as the campaign to leave the European Union is known. Given the depth of the fissures, the wounds seem likely to fester for years.
Much of the debate hinges on the unknown — the fear of it for some, the thrill of it for others. Released from the shackles of membership in the bloc, pro-Brexit campaigners argue, Britain will embark on a golden age built on its long and doughty history as a trading nation.
Not so, their adversaries say: Outside the European Union, Britain will confront falling living standards, deeper insecurity and rising uncertainties.
Beyond that, though, the arguments pit head against heart. A vote either way is a leap of faith. Indeed, the Church of England has raised a cautious banner with a newly minted prayer for “honesty and openness,” “generosity” and “discernment” in the deliberations.
The fact that Mr. Obama spoke out determinedly for Britain to remain in the European Union was perhaps the biggest omen for Britons that their international stature may be in jeopardy.
Outside the bloc, Mr. Obama said, Britain would be “at the back of the queue” of those seeking trade agreements with the United States. His intervention stirred passions of its own.
“Americans are rebelling against the emergence of an imperial presidency,” the columnist Tim Montgomerie wrote in The Spectator, asserting that as Mr. Obama “lectures the British on their place in the world, voters here might feel somewhat resentful, too.”
Some suspected that Mr. Obama’s support was a maneuver as part of murkier negotiations on a contentious trans-Atlantic trade pact.
Britain has long sought to assure itself that it has a “special relationship” with the United States, built on shared history, values, language and culture.
But the president’s remarks, said the columnist Jonathan Freedland in The Guardian, signaled “that America has no intention of forming some new, closer relationship with Brexited Britain.”
For those arguing that “a brighter, non-European future beckons,” Mr. Freedland said, “Obama burst that bubble.” What makes the debate so passionate and all-absorbing is that life for Britain outside the European Union — much as life beyond the looking glass — is terra incognita.
And, as Alice discovers, the answers rarely match up with the questions. “The more this debate has continued,” Nick Clegg, a former deputy prime minister, said during a public debate, “the more I have the feeling this debate is about who we are, who Great Britain is.”
“This is not just about what you think of Brussels. It’s also what we think of ourselves.”