British history: State of the unions Acts of Union and Disunion. By Linda Colley. Profile Books; 225 pages; £11.99 . ON SEPTEMBER 18th 2014 the people of Scotland will vote on whether to become an independent country. The choice will depend partly on individuals" hopes and fears about their future as part of a separate state. It will rest also on their vision of 300-plus years of union. What has held the nations of the United Kingdom together for so long - and what is now pulling them apart - is the subject of Linda Colley's short and fascinating study, "Acts of Union and Disunion." The book centres on Britain's internal and external relations but the themes it explores are universal: how national identities react to globalisation and migration. Ms Colley's central thesis is that, like all countries, Britain is a synthetic construct. It is an "imagined community" as Benedict Anderson, a social scientist, defined nationhood: people are bound not by face-to-face interaction but a communal sense of project and purpose, maintained by institutions. "In order to succeed and flourish, states and nations need an attractive idea of what they are," she writes. The question is whether the idea of nationhood is still "attractive" enough to keep the distinct parts of the "United" Kingdom together. Britain, says Ms Colley, is "not all that old in fact." However, a number of myths have helped to keep it unified. One is the idea of Britain's "liberty-drenched ancient past." This began with the Magna Carta, a document that was signed in 1215 in an attempt to limit the monarch's power and included such terms as "free man." The fable prevailed at home and abroad; it helped justify Britain's colonial ambitions, for example. Another useful story is that of "islandhood," as though spanning a geographical territory from sea to sea was manifest destiny. Yet from the early Middle Ages until the 19th century Britain - anyway a compound of islands - shared monarchs with Europe, and cultural and religious references too. Exchanges across the sea influenced the country's identity in other ways as well, from its 18th-century naval dominance to the maritime trade and imported tea, cotton, tobacco and other goods that shaped habits and lifestyles. Now, in an age of mass air travel, islandhood and the sea's proximity are ever less relevant. But as Ms Colley reminds the reader, stories need not be accurate to be influential. Each of England, Ireland, Scotland and Wales recounts its own binding history. Yet Ms Colley suggests that England, the dominant part of the union numerically and linguistically, is the least well defined, and notions of what it means to be English often divide rather than cohere. What most think of as "England" is "bottom-heavy": wealth, power, population and cultural institutions are weighted towards the south, particularly "over-mighty" London. By contrast the north, a "very large and scenically variegated" area, lags in health, income and education. Ms Colley's sobering conclusion is that fear, not hope or friendship, has kept Britain together. Each union was forged in a time of conflict with Europe. By contrast, "periods of protracted peace have repeatedly presented the most profound threats." Although Ireland broke away during the first world war, most demands for looser union have followed calmer interludes. Scotland's referendum on independence lies within this context. Ms Colley, who has long studied the complexities of British national identities, purports to be neutral. Yet she is not, quite. For the prescriptions she offers are designed to maintain some kind of union. She calls for England to have its own parliament, for local governments to raise and collect more taxes and for Britain to have a written constitution. This last, she believes, "might supply some fresh constitutive stories for a new kind of union." Whether Britain's politicians could write this new magna carta is less clear.