It cannot have occurred to many American loyalists in November 1783 that they were fortunate people. When war erupted between Britain and its 13 North American colonies in 1775, a fifth (possibly as many as a third) of American colonists, as well as large numbers of indigenous peoples and free and enslaved blacks, had aligned themselves in support of a continuing transatlantic imperial and regnal connection. Many of these loyalists subsequently died in battle, or skirmishes, or from wartime epidemics. Others experienced a catastrophic loss of status, or saw their property confiscated; and some lost much more. Thomas Brown, who had arrived in Georgia from Yorkshire in 1774, refused to join the "rebels" the following year and was promptly set upon by a gang who tortured and almost scalped him. Shadrack Furman, a free black Virginian who worked for the British as a provisioner and guide, was captured by patriot troops, given 500 lashes, and then blinded and rendered mentally deranged by an axe blow to the head. A war involving high political ideals, this was also  as civil wars generally are  bloody, arbitrary and extremely cruel. And now Britain had finally conceded defeat, and the last of George III's troops were abandoning New York.

For a very long time, loyalists were often left out of patriotic American histories of the revolution. Or they were caricatured as upper-class Tory reactionaries, or  rather like the Jacobites  made the subject only of nostalgic antiquarianism. Maya Jasanoff's achievement in this vivid, superbly researched and highly intelligent book is skilfully to weave together and supplement a mass of recent revisionist research on these men and women, and to analyse their complex roles and significance in the imperial and global history of the late 18th and early 19th centuries.

The contents page of Liberty's Exiles begins with a "List of Maps" and a "Cast of Characters": aptly so, because Jasanoff organises her book around different geographical regions and around the stories of diverse individuals who all experienced some measure of displacement. Most loyalists remained in the United States after independence was won, reintegrating into society there, or not. But about 60,000 loyalists, whites, blacks and Native Americans, left the new republic, and some 15,000 slaves went with them: in all, an exodus of one in 40 of the American population in 1783.

Most of these exiles settled in Nova Scotia and New Brunswick, with a further 6,000 going to what was then Quebec. Some 8,000 whites and 5,000 blacks sailed to the British Isles; another 5,000 loyalists went to the Bahamas, where the British government made available free land. Still more refugees travelled initially to East Florida, to Jamaica or to Central America. Some even ended up in India and in what became Australia. Yet, for many, it was not a case of a one-off migration. Movement became a habit and  sometimes  an opportunity. Elizabeth Johnston, who married a loyalist army officer and medical student in 1779, spent almost half of her long life on the move, shifting from Savannah to East Florida, before seeking refuge in Edinburgh, then in Jamaica, and finally in Nova Scotia, losing four children and her husband along the way. Conversely, 500 black loyalists who escaped initially to Nova Scotia chose in 1791 to join a British expedition aimed at establishing a free black settlement at Sierra Leone, a planned new promised land. Starved of hope and respect in frozen Nova Scotia, just as he had been in what was now the United States, one of these volunteers declared he would rather take his chance on Africa and "die in my own country than this cold place".

Many of the black loyalists in Sierra Leone did die, from disease, hunger, failed rebellion, or French attack. But the fact that these men and women were given this chance to relocate to Africa, after first migrating through North America, points to the profound ambivalence of the loyalists' situation. At one level, they were the defeated: people who had ended up on the losing side of a major conflict. But at another, crucial level, the losing side in the American revolutionary war continued to be the world's largest and most powerful empire. So while loyalists could no longer hope to live as faithful subjects of the British monarch in the onetime 13 colonies, options to remain British subjects in another place remained available to them in every continent of the globe. By seizing on these options in different ways, as Jasanoff describes, itinerant loyalists helped in the 1780s and after to shape a regenerated British empire.

To be sure, some loyalist imperial projects foundered. In the Bahamas, loyalist incomers exacerbated racial divisions, and failed in attempts to introduce large-scale cotton plantations. Various loyalist schemes to reconquer parts of the US on behalf of the British also failed. William Augustus Bowles, a flamboyant Marylander who was married to a Creek, never got the chance to found Muskogee, a loyal but independent Native American state he envisaged situating in the midst of Florida. In other regions, however, loyalist agency proved decisive. In particular, their migrations served to transform the demographic, religious and political make-up of what is now Canada.

Exploring imperial and global history through the lens of particular families and individuals in motion is becoming a well-established methodology, and Jasanoff employs it with terrific panache. But close attention to the highly mobile should never obscure the power and prejudices of those very many people in the past who remained still and entrenched. As Jasanoff recognises, by the early 19th century, opportunities for crossing boundaries and switching allegiances were receding in the face of increasing nationalism and state controls. In retrospect, the years between the end of the American war in 1783 and the outbreak of Britain's war with revolutionary France in 1793 were an extraordinary period of flux and opportunity, and some loyalists were able to exploit this to the full. In that respect, too, they were more fortunate than they knew.

Linda Colley's Britons: Forging the Nation 1707-1837 is published by Yale.