Like the much-mythologised Wild West, the Debatable Land was terra nullius, where the writ of law was by and large ignored and bad men roamed at will. Situated where the northwest of England meets the southwest of Scotland, it was long an inhospitable, inhuman corner of the country: hilly, boggy, inaccessible and, five centuries and more ago, a tapestry of trees. The weather was similarly forbidding. When it rains in these parts, which it does more often than not, it is with torrential relentlessness.

Why anyone with a choice in the matter would want to live hereabouts is not easily explained. Graham Robb, whose books include biographies of Balzac, Hugo and Rimbaud as well as an innovative history of Paris, moved in 2010 from cloistered Oxford to a pile in the middle of nowhere. One of its former owners was Nicholas Ridley, of Northumbrian stock, who as a minister in the Thatcher government was responsible for launching the poll tax at his Scottish neighbours. As Robb writes, ‘The fact that Ridley had settled on the border itself was a kind of provocation, as was the title he chose for himself when he was created a life peer: Baron Ridley of Liddesdale.’

The main tourist attraction of Liddesdale, at the heart of the Debatable Land, is Hermitage Castle. Sir Walter Scott, an avid collector of Border lore and ballads, was spot on when he described it as ‘that grim and remote fastness’. Even on the sun-blessed day when I visited, it exuded menace and malice, the kind of place where torturers could remove toes and tongues without fear of dissent from the neighbours. It was to this godforsaken spot, Robb recalls, that in the autumn of 1566 the 23-year-old Mary, Queen of Scots, rode twenty-five miles from Jedburgh to meet her lover the Earl of Bothwell.

The earl was recovering from a near-fatal sword wound delivered by an infamous reiver called Little Jock Elliot. The Elliots were perhaps second only to the Armstrongs in venality. Both would have made mincemeat of the Sopranos. It was Bothwell’s task to temper their rapacious thievery and thirst for violence. By venturing into their midst, Mary, who obviously did not lack courage, was taking a considerable, if calculated, risk. Luck, however, was on her side. Recently, Bothwell’s troops had slaughtered a number of Armstrongs and Elliots, ‘and it was twelve days yet to the full moon, when reivers were at their most active’.

Such diverting asides animate Robb’s revelatory account of this oft-overlooked and understudied part of the United Kingdom. Previously, the must-read book on the subject was George MacDonald Fraser’s The Steel Bonnets, published in 1971, which, as might be expected from the creator of Flashman, did not lack panache. Robb’s approach is no less appealing, a combination of personal colour and academic rigour.

He and his American wife, Margaret, are cyclists rather than drivers. They see the countryside in which the reivers wreaked havoc from a perspective denied those stuck behind a wheel. Robb writes well and on occasion poetically, as, for example when he compares the sheep that cover the hillsides to ‘giant daisies’ and the reivers to ‘a creeping fungus’. He is not inclined to romanticise the latter. The question that keeps recurring to him is why, if the Debatable Land was ‘such a hell on earth’, did anyone live there? The short answer is that they had no alternative; the longer one is rather more complicated.

For good or ill, this was their home and reiving was less an opportunistic, atavistic activity than a way of life. It was part of the annual cycle in which raising and rustling cattle went hand in hand. Attempts were made by rulers on both sides of the border to temper the reivers’ abominations but most failed. The reivers owed their allegiance not to any country or cause but to their immediate clansmen. Indeed, Robb convincingly argues that, in contrast to ‘state-sponsored violence’, reiving ‘maintained rather than wrecked the social fabric’.

One ‘notorious’ instance of cross-border relations occurred in 1599, when reiving was at last beginning to peter out. The Armstrongs took on the Ridleys in what MacDonald Fraser wrote was ‘the fore-runner of the Scotland v England internationals’. But what was meant to be a six-a-side football match turned into a bloodbath when the Armstrongs, realising they were about to be ambushed, reacted as only they knew how. Three Englishmen were killed, thirty were taken prisoner and many more were ‘sore hurt’, including one poor fellow whose bowels spilled out. On that score, Baron Poll Tax could count himself fortunate.

As Robb pedals and buses around this grim terrain, he encounters a number of unforgettable, if not insouciantly violent, characters. One such is a venerable mole catcher called Wattie Blakey, whose job it is to exterminate the subterranean pests. Blakey’s success rate is there for all to see, his quarry ‘hooked by the snout onto barbed-wire fences, like sacrifices to a local god’. Like Robb, the mole-catcher doesn’t drive, which means he must rely on the less-than-perfect local bus service to convey him to far-flung parts of his bailiwick, where he has placed seven rusting bicycles that he uses to take him even further off the beaten track. Like the reivers of yore, Blakey is wedded to his profession, which he is no more able or inclined to separate himself from than they were from theirs.

The Debatable Land ends with a brace of discoveries. The first is a key to understanding Ptolemy’s second-century map of Britain, hitherto thought inaccurate, which will surely be invaluable to future historians. The second is the earliest account told from a British point of view of a major battle in these islands. This is all fascinating. But of much more interest, surely, is Robb’s controversial assertion – made in a throwaway parenthesis – that haggis, far from being a Scottish invention, is in fact English in origin and that its current status as Scotland’s national dish is due entirely to Robert Burns’s 18th-century paean to it. This is dangerous stuff and Robb, who is clearly no fan of Scottish nationalism and knows better than anyone what northern Britons are capable of, would have been wise to have kept such an incendiary ‘fact’ to himself.