LATER THIS year, the United Kingdom could disunite. In September, Scotland is due to vote on whether to become an independent nation. There is a strong chance that the Scots will vote to go it alone, breaking a political union with England that was established over three hundred years ago, through the Act of Union of 1707. The Scots number only 5.3 million of the United Kingdom’s population of 63.7 million. But Scotland accounts for a large amount of Britain’s territory and coastline—and contains several of the nation’s finest universities, castles and golf courses. Moreover, Scotland is also where Britain’s nuclear weapons are based, and the country’s (dwindling) oil supplies are almost all located in Scotland’s coastal waters.

Americans who have not noticed that the United Kingdom might be about to break up can be forgiven. Even in England, many citizens are only just waking up to the idea that the nation they are living in might go poof later this year. When the UK government led by David Cameron agreed in 2012 that a referendum on Scottish independence would be held, it was widely assumed that the result would be a foregone conclusion. And it remains true that in the scores of opinion polls that have been taken since then, not one has yet shown a majority in favor of independence. Yet, earlier this year, the polls began to narrow. Several recent snapshots of public opinion have shown the gap between the “Yes” and “No” camps to be down to three to six percentage points.

There is also a discernible gap in the energy and optimism of the two campaigns. On a brief trip to Edinburgh earlier this year, I decided to try to visit a proindependence event and a prounion meeting. The Yes campaign held three meetings in the area over the course of two days. The No camp, however, seemed to have only two events scheduled—for the entire month. The proindependence camp also has a network of eager enthusiasts, which is expected to mount an effective “get out the vote” campaign. A Scottish journalist told me that if the proindependence camp managed to narrow the gap in the polls to three points by the time of the ballot, he expected they would emerge victorious in the actual vote—simply on the basis of their superior organization. So there is now visible nervousness and squabbling among the pro-UK forces. For one thing, Cameron is starting to realize that he may be seen as the feckless prime minister who presided over the loss of Scotland—giving him a place in the history books alongside Lord North, who lost the American colonies. In that event, he would surely feel compelled to resign as prime minister the day after a referendum defeat.

HOW DID it get to this point? What are the implications for Britain and the wider Western world? Any search for the origins of the current drive for Scottish independence must start by acknowledging the obvious fact that Scotland was independent of England for much of its political history. The monarchies of the two nations were unified in 1603, and a formal political union was agreed upon in 1707 only after a significant financial crisis threatened to bankrupt Scotland. Even after the political union was consummated, Scotland provided the base for two Jacobite invasions of England in 1715 and 1745. As a result, Scottish nationalism has a rich history of battles against England from which to create a national story that might justify independence. Helpfully, the referendum has been scheduled for the year of the seven hundredth anniversary of the Battle of Bannockburn, Scotland’s most famous victory over England. (An American friend of mine who went to a Scotland-England football match was first baffled, then awed, to see Scotland supporters carrying a banner that said “Remember 1314.”)

During the nineteenth and twentieth centuries, these historic antagonisms were widely assumed to have been buried, as the English and Scots united around common enterprises—most notably the construction and management of the British Empire. The Scots played a prominent role as explorers, missionaries and imperial administrators. The threat of invasion from the Continent—first by France, then by Germany—and the experience of fighting together in the world wars also served to unite the English and Scots. In retrospect, the end of the empire after decolonization in the 1950s and 1960s probably weakened one of the central pillars of the common British identity. Moreover, the rise of the European Union provided an indirect boost to nationalism. The EU now comprises twenty-eight members. The Scots see smaller nations such as Ireland and Slovenia with an honored place at Europe’s top table, and conclude that the EU has made it viable to be a small and prosperous nation, sheltered under the European umbrella. The EU, along with NATO, is widely assumed to provide an answer to the security concerns of small European nations, although the predicament of the Baltic states may soon put that proposition to the test.

The advent of Thatcherism in Britain in 1979 provided the potent rocket fuel of resentment that is so crucial to the success of any nationalist movement. Margaret Thatcher presided over the closure or shrinkage of many of Britain’s struggling industries. Enterprises like Scotland’s steel mills, mines and shipyards were a central part of the country’s identity. Economic change probably meant that they faced a bleak future under almost any government. But mass unemployment in Scotland at the hands of a Conservative government with its political base in southern England allowed Scottish nationalists to portray their nation’s economic problems as the product of a deliberate act of class warfare by an unsympathetic, upper-class English government.

The rise of the Labour Party government in 1997—led successively by two prime ministers who grew up in Scotland, Tony Blair and Gordon Brown—seemed to hold out the promise for a renewal of the union. New Labour set up a devolved Scottish government, with considerable powers over policy areas like education and health, in a move that was intended to neutralize Scottish grievances. It didn’t. Instead, the Scottish Labour Party was itself increasingly moribund and uninspiring, with its key figures looking for careers in London and its local party notorious for machine politics. In 2011, the Scottish National Party (SNP), whose raison d’être had always been independence, defeated Labour to take control of the Scottish government in Edinburgh. At that point, the UK government felt obliged to meet the demand of the SNP for an independence referendum.

Fatefully, by this time, the government in London was once more led by a Tory prime minister who himself was something of a toff. The leaders of Thatcher’s party, with just one member of Parliament of their own in Scotland, were peculiarly ill placed to argue the case for the union in Scotland. Cameron has deliberately avoided campaigning in Scotland, tacitly acceding to the nationalists’ case that the government in Westminster lacks legitimacy in Scotland. He has also compounded long-standing problems in the relationship between England and Scotland with some serious blunders. Initially, the SNP campaigned for a third choice to be placed on the ballot—neither independence nor the status quo, but a further transfer of powers to the Scottish government in Edinburgh, while retaining Scotland within the union. Opinion polls suggested that this option, which went by the unlovely name of “devo max,” was the most popular. But Cameron balked. He refused to put it on the ballot. He apparently believed that a binary choice would allow the UK government to score a decisive victory over Scottish nationalism. This calculation, however, increasingly looks like a reckless gamble. Cameron’s second error was to fail to insist that the eight hundred thousand or so Scots residing in England should get a vote in the Scottish referendum. This deprived the strongest unionist constituency of a vote in the Scottish referendum.

CAMERON MAY rue these moves, come September. The English are still some way from imagining how their country would feel the morning after Scotland had voted for independence. But the likely reaction would be anger and incredulity. The country would immediately face some important symbolic and substantive questions. Could it still be called the United Kingdom of Great Britain and Northern Ireland if it loses part of the island of Great Britain? Would the United Kingdom have to redesign its flag, the Union Jack—which currently contains the Scottish cross of St. Andrew (the blue bit)?

Other, more substantive issues would quickly come into focus. The Irish question would be reopened, as Northern Ireland’s status within the United Kingdom began to look increasingly anomalous. (This is one reason, incidentally, why the government of Ireland quietly dreads the prospect of Scottish independence.) What would happen to the pound? Paradoxically, the SNP—in a bid to reassure voters—has said that it would like to retain the pound. Downing Street, however, has ruled out a currency union.

This is not all. What would happen to the totems of British power and international status, the permanent membership on the UN Security Council and the nuclear weapons? The UN role would probably pass straight to the continuing government in London, much as the Russian Federation inherited the Soviet Union’s Security Council membership.