EDINBURGH, Scotland — What does it profit a country to win its independence only to lose itself? Of all the imponderables arising from the United Kingdom’s decision to leave the European Union, this is the most significant, the most far-reaching and ultimately the one that will help determine whether Brexit is a great success or the most Pyrrhic victory in recent political history.

Because at stake is the future of Britain itself. “Brexit means Brexit,” says Prime Minister Theresa May. It may also mean the end of Britain. The general election, to be held June 8, is likely to see May returned to Downing Street with a significantly increased majority. That, she says, will strengthen her hand in the forthcoming Brexit negotiations.

The opinion polls report that the Conservatives enjoy a huge lead over the Labour Party; losing from this position would be an act of unprecedented carelessness. The longer-term threat to May, however, comes from the north, not from Labour. The Conservatives will win handsomely in England, but in Scotland, the pro-independence Scottish National Party is likely to win another crushing victory that will confirm the sense that there are two distinct elections happening in June: one in England, the other in Scotland.

Last month, the Scottish parliament passed a motion authorizing the SNP to press the British government to hold a second referendum on independence. May, preoccupied with Brexit, refuses to accommodate the Scots’ request, declaring that “now is not the time” for a second plebiscite that might finally and definitively answer the national question that has dominated Scottish politics for years. She has no intention of risking independence and being remembered as the prime minister who presided over the breakup of the United Kingdom after 310 years of union.

But if the SNP wins the Scottish portion of the election — the party holds 56 of Scotland’s 59 constituencies in Britain’s Parliament — its mandate for a second tilt at independence will be greatly strengthened. That leaves May in a desperate quandary: How long can she frustrate Scottish claims to self-determination without stoking the separatist fires she seeks to extinguish? Can she say no forever?

The prospect of a Spanish-style standoff looms. Madrid refuses to recognize Catalonia’s demands for independence, denying the Catalans the right to a legal referendum that would decide the region’s constitutional status. That has done little to dampen Catalan aspirations for independence and even less to settle Spain’s constitutional question. Something similar may be said of Britain and of Scotland.

No one can claim they were not warned. In April last year, a senior member of what was then David Cameron’s British government gave a speech warning that, among the many uncertainties that would follow a vote to leave the European Union, one should concentrate minds more fully than any other: A vote for Brexit risked the integrity of the United Kingdom itself. Brexit would not be fatal to the E.U., but “we might find that it is fatal to the union with Scotland.” Noting that Scotland seemed likely to vote to remain in the E.U., the senior cabinet minister warned that “if the people of Scotland are forced to choose between the United Kingdom and the European Union, we do not know what the result would be.” The stakes could hardly be higher: “I do not want to see the country I love at risk of dismemberment.”

That cabinet minister was Theresa May, then serving as home secretary. Now, as prime minister, she is charged with proving her own fears mistaken or, at best, exaggerated. Her reputation — to say nothing of her country — rests upon the success or failure of her attempt to hold the United Kingdom together. Scottish independence, rejected in a referendum less than three years ago, is back on the agenda.

It has been placed there by May’s own party. The Conservative and Unionist Party’s enthusiasm for Brexit imperils the union that is, notionally at least, a core and foundational element of the party’s identity. Last April, May argued that she did “not want the people of Scotland to think that English Euroskeptics put their dislike of Brussels ahead of our bond with Edinburgh and Glasgow.”

But they do. No other interpretation is possible. A recent survey of Conservative members found that only one in three thought that Scottish independence would be a grievous blow to Britain and its sense of self. Nearly 30 percent actually welcomed the idea of Scotland seceding from the U.K. The United Kingdom, it is now clear, can be imperiled in England as well as in Scotland. English indifference leaves Scotland’s Unionists increasingly isolated; England’s vote to leave the E.U. makes it easier for Scottish nationalists to argue that Scotland and England no longer share a common political culture or a shared vision for the future. That’s a gamble the Tory party seems prepared to make.

Brexit — which May opposed but now says is a tremendous and exciting opportunity for Britain — has handed the pro-independence Scottish National Party, the dominant force in Scottish politics for the past decade, all the excuse it needs for a fresh push toward independence. Brexit’s difficulty is Scotland’s opportunity. The question for Scots is whether the increasingly evident risks of Brexit justify a second leap into the deep, dark waters of the unknown.

Across the United Kingdom, 52 percent of voters supported leaving the E.U., but in Scotland 62 percent voted to remain. Hours after the Brexit result was confirmed, Nicola Sturgeon, leader of the SNP and Scotland’s first minister, declared that independence was again “on the table.” Scotland’s future must be decided in Scotland, not by the preferences of English voters.

Echoing last year’s “Leave” campaign, the SNP argues it is time for Scotland to “take back control” of its own destiny. The 300-year-old union with England no longer serves Scotland’s distinct interests and, with Scots outnumbered 10 to 1 by the English, is incapable of reinventing or renewing itself to take account of Scotland’s pro-E.U. sentiment. The Scottish government’s proposals for a “differentiated” Brexit deal which would have maintained Scottish membership in the E.U.’s single market were rejected by May’s government.

Although there is a pro-independence majority in Scotland’s devolved legislature, the power to hold a legally binding referendum rests with the Westminster government. Sturgeon must seek permission for a fresh plebiscite, permission that has been refused. May, whose government is consumed by Brexit, has no wish to fight on a second constitutional front. “Now is not the time” for a second independence referendum, she says, although this implicitly concedes that there may be an appropriate time for a second independence poll at some point.

Her government, however, intends to delay that moment of reckoning for as long as possible. It would be perverse, it argues, to hold a referendum in response to Brexit before the full terms and conditions of Brexit have been determined. Sturgeon desires a vote in late 2018 or early 2019 as the details of Britain’s exit from the E.U. become clear; May is determined that no referendum will be held until the impact of Brexit has been felt. If Brexit is a success, U.K. government sources think, Scottish enthusiasm for independence will be diminished.

If this is a mighty gamble for May, Sturgeon is also playing a game heavily freighted with risk. Falling oil prices and a stagnant Scottish economy mean that an independent Scotland would begin its existence saddled with a deficit of close to 10 percent of gross domestic product. That would require urgent attention, demanding spending cuts and tax increases. The Union long guaranteed economic security, but this, too, seems less certain in a post-Brexit environment. Even so, independence must be a leap into the unknown, a voyage of national rediscovery that is pregnant with uncertainty.

For instance, it is not clear what currency an independent Scotland would use; for that matter, the precise relationship between Edinburgh and London as well as between Edinburgh and Brussels remains a matter of conjecture. Joining the E.U. as a newly independent state would come with terms and conditions, not all of which might prove palatable to Scotland. Sturgeon says she would wish to rejoin the E.U., although that leaves open the question of how the border with England — an E.U. frontier in such circumstances — would be enforced. Few Scots are enthused by the idea of customs posts on the Anglo-Scottish border. And if, as Sturgeon argues, leaving the E.U.’s single market is economically “ruinous,” how expensive would leaving the U.K.’s single market be for a Scottish economy that sells four times as much to the rest of the U.K. as it does to the E.U.?

In 2014, Scotland voted clearly but not definitively in favor of maintaining the integrity of the United Kingdom; 55 percent of voters rejected independence. The Unionists may have won the battle, but they lost the peace. The SNP won a landslide victory in parliamentary elections the next year, taking 56 of Scotland’s 59 Westminster constituencies. Labour, traditionally Scotland’s leading party, lost all but one of its MPs as once-loyal Labour voters who backed independence deserted the party in a wave election that also realigned Scottish politics along constitutional lines. At the time, Sturgeon accepted that absent a “material change in circumstance” there could be no reasonable grounds for a second referendum. Besides, the SNP would prefer to launch a campaign for a second referendum at a time when polls suggested most voters favored independence. Such a plebiscite would be one to confirm, rather than decide, Scotland’s collective opinion.

Brexit changes and complicates everything. It makes the democratic or political argument for independence clearer and perhaps more persuasive; it also makes the practicalities of independence more difficult than ever. Will Scottish voters really respond to one confusing constitutional drama by embracing another? Or will risk aversion diminish Scotland’s enthusiasm for separatism?

Opinion polls suggest that Scotland remains a country almost evenly divided — and sharply polarized — by its views on the national question. May and Sturgeon — the political Queen of the South and the Queen of the North, respectively — have staked their reputations, their careers and their legacies on the still-great and unanswered matter of Britain. Only one of them can prevail. Brexit may yet break Britain; if it does, then it will be little consolation to May that she had it right once.