Hugh Thomas was Mrs Thatcher's Falklands mentor - but now the great Hispanic historian belongs to a party of one. Richard Gott follows his campaigns
Scratch a historian and you usually find a politician manque. Tickle a politician, especially in the heritage atmosphere of the House of Commons, and you often uncover an amateur historian. Most historians with Labour inclinations ignore Westminster and stick to radical politics outside, often refusing the honours that might be their due. (Note to Tony Blair: Lord Hobsbawm? Sir Basil Davidson? Christopher Hill OM? Surely a radical gesture within cash limits). Tory historians, on the other hand, enter national politics in droves: Alan Clark, Ian Gilmour, Robert Rhodes James, Robert Skidelsky and, of course, Lord Thomas of Swynnerton, ennobled by Margaret Thatcher in 1981.

Hugh Thomas has been something of a cuckoo in the Tory nest for two decades. His writings suggest he is one of the surviving romantic historians of an earlier, liberal school. He likes old-fashioned, dangerously European words like "liberty", which often sound as if translated from the Spanish or the German. Yet in one sense he stands in a conservative tradition. He has an unfashionable eye for the epic struggle: the Spanish Republic against Franco; the Mexicans against the Spaniards; the Cubans against Spain and the Americans. His history springs from a deep belief that we ought to know about the past to avoid mistakes in the future. No postmodernist fictions for him.

Now Thomas is about to publish another of his great blockbusters, those studies over nearly 40 years that have oscillated across the Atlantic from shore to shore. Almost alone among historians of his generation, he has put the Hispanic world on the map for British readers. First, he concentrated on Franco and the Spanish Civil War. Then, inspired by Castro's revolution, he turned his attention to Cuba. Soon he was back in Spain again, writing books on Goya and Primo de Rivera. Then he recrossed the Atlantic to Mexico, and a book on the Spanish conquest. Now, still on the transatlantic theme, he has produced an immense work on the slave trade embracing the history of the two continents that interest him most - and the waters between them.

The Slave Trade: the history of the Atlantic slave trade, 1440-1870 (Picador, pounds 25) is a liberal version of the theme to rival the two-volume study completed this year by the Marxist historian Robin Blackburn (for Verso). Where Blackburn concentrates on the economic underpinning of slave society, and its inherent racism, Thomas is more interested in the people and personalities of the Atlantic trade. And he takes a perverse delight in emphasising the entrepreneurial role of African and Islamic rulers in it.

Both these two great studies are by historians outside the academic mainstream. Blackburn edits the New Left Review, while Hugh Thomas is essentially a freelance who investigates subjects that intrigue him. His Unfinished History of the World dealt in part with such "neglected topics as the history of brandy, the thermometer and the radish".

Although Thomas has briefly served as a university historian (a professor at Reading in the Sixties) and makes guest appearances on the academic circuit (he is now working in Boston), he remains basically unattached. He used to describe himself as "a historian in private practice", as if he were a solicitor or a doctor to whom you could turn for advice.

Such a rolling-stone attitude has not endeared him to the establishment. As a distinguished historian of Spain and Latin America, famous on both sides of the Atlantic, he has been awarded no chair in Britain's older universities. Nor has he run any of the institutes dedicated to Latin America that proliferated after the Cuban Revolution. There is no "Thomas" school of history, no acolytes tugging at his coat-tails - though a few students remember with enthusiasm his seminars at Reading on anarchism during the Spanish Civil War.

Yet he always appears to enjoy his independent existence. He has lived many lives over the years and now, at a youthful 66, he still looks as though he might reinvent himself all over again.

Although he published his masterwork on the Spanish Civil War when he was not yet 30, Thomas started life in a political rather than historical mould. President of the Cambridge Union in 1953, he went on to work at the Foreign Office, and was briefly a prospective Labour candidate (for Ruislip-Northwood). Doubtless, had he consigned himself to 40 years of mind-numbing tedium, he might be Tony Blair's Foreign Secretary, following in the footsteps of Douglas Hurd (president of the Cambridge Union in 1952). But it would have been a high price to pay.

For many years, Thomas survived as an archetypal social democrat, a close friend of Roy Jenkins and a biographer of John Strachey. His wife Vanessa is the daughter of a diplomat and Liberal peer, the late Lord Gladwyn. His early work on Spain turned him into a passionate European. Thomas would have been a useful spare wheel, when Labour splintered in the 1980s, for the Gang of Four, the spiritual home of his friend and Notting Hill neighbour Anthony Sampson.

Yet by that time Thomas had long grown bored with social democracy. Already in the 1970s, along with his old friend Paul Johnson, he had made the dramatic leap into the proto-Thatcherite camp. While once he might have aspired to be a Regius Professor, he now thrust himself into the small world of the think tank. In 1979, he became chairman of the Centre for Policy Studies, the creation of Keith Joseph and Alfred Sherman designed for thinking the unthinkable. (One unthinkable result of Thomas's arrival was the defenestration of Sherman, a fellow Hispanicist and veteran of the International Brigades, but no fan of Lord Swynnerton.)

From the CPS it was but a short step to Downing Street. Thomas, with his Latin American experience, soon found himself holding Mrs Thatcher's hand during the Falklands War - a story he has yet to tell.

Maybe Thomas the politician has been satisfied by this experience. Politically, he is now reduced to a party of one. He is that unusual construct, a Thatcherite Europhile, a contradiction in today's Britain though perfectly possible in the Hispanic world that he writes about. He looks anxiously over at the cross-benches of the House of Lords, well knowing that there oblivion lies.

When politics are dull, there is always history to be written, though research for the independent scholar is not so pleasurable as it used to be. To his eternal regret, one of his campaigns ended in failure. In 1983, at the height of his influence, he published an impassioned plea for the British Library to be allowed to remain in its historic place within the British Museum. He was better positioned than any other scholar to resist the move. But the bureaucratic juggernaut moved on, and the Reading Room closed in October. Now, like everyone else, he will have to make the dismal trek to St Pancras.