Conquest: The English Kingdom of France, By Juliet Barker

The books that first brought Juliet Barker renown were moving studies of the Brontës' lives and letters, and an immense life of Wordsworth. Her reinvention as a medievalist with her last book, Agincourt, seemed extraordinary, but in fact the interest in heraldry and chivalry predated her appointment as curator at the Brontë Parsonage Museum. Agincourt was a thrilling read, immensely informative and eye-opening. Henry V was a painstaking accountant, and rarely have military shopping lists made such exciting reading.

Agincourt ended on a high, but by page 45 of the follow-up, Henry V lies dead, probably of dysentery, and the story of the "English Kingdom of France" has lost one of its most fascinating characters. So what happened after the Agincourt campaign of 1415?

Although the story arc – glory, defeat, hypocrisy, incompetence, disaster and shame – is less uplifting, at least to English readers, this book is as gripping as its predecessor.

The English Kingdom would never have lasted as long as it did without deep divisions in France itself. Charles VI's madness (he believed he was made of glass) created a power vacuum; loyalties were divided between the Armagnac and Burgundy factions. The hostile and mutually suspicious factions, in the persons of John the Fearless, the duke of Burgundy, and the dauphin, met in 1419 to discuss unifying to repel the English. As Burgundy knelt defenceless before him, the dauphin's retainers hacked him down. A churchman later commented that it was through the hole in Burgundy's skull that the English entered France.

The most famous character on this stage after Henry V is probably Joan of Arc (here styled Jehanne d'Arc). Barker is not writing women's history: this is a tale of warlords and ruthless killers, and her handling of Jehanne's story is, if not debunking, certainly rather cool. The dauphin, crowned Charles VII at Jehanne's behest, comes across as an appalling character, devious and unprincipled. Barker shows that Jehanne was merely a useful propaganda tool, to be jettisoned once her apparently miraculous abilities had waned. She was not even unique: there were several divinely inspired "prophets" around Charles's court. Another, Guillaume le Berger, was captured and paraded in Henry VI's coronation procession, bound with rope. Barker chillingly adds, "He would disappear at the end of the day, supposedly having been thrown in the Seine to drown."

Along with the dodgy dauphin and La Pucelle ride a host of fascinating characters, who, whether they appear for a few chapters or only a few lines, are brought to life by Barker's pen. There's the Pimpernel-like La Hire, Charles VII's brilliant military strategist, uncatchable and seemingly unkillable; John, Duke of Somerset, who endured 17 years as a French hostage and upon release hurled himself into the fray, covered himself with ignominy on the battlefields of France and went back to England a ruined man; the elderly earl of Warwick, who reluctantly accepted a high military post that was "full far from the ease of my years, and from the continual labour of my person at sieges and daily occupation in war".

There are even those who continue to play a role post-mortem, such as the earl of Arundel, shot in the ankle in battle. His leg was amputated in a desperate attempt to save his life, but he died anyway, aged 27. His body was recovered from enemy hands by an enterprising hostage-taker and ransomed back to his family so he could be buried in the family vault. And spare a thought for the nameless French knight who, taking refuge in a hollow tree after an ambush in 1430, got his armour stuck fast and was discovered only when the tree was cut down in 1672.

The story is rife with brigands, pirates, man-eating wolves (even in Paris), child brides, spying monks and a legion of men called simply Bastard (there's the Bastard of Vendôme, the Bastard of Orléans, and two very unpleasant Bastards of Bourbon, known as "les écorcheurs" – the flayers. Perhaps it's not surprising, then, that even a bizarre figure such as Gilles de Rais, soldier, sorcerer and sodomising mass-killer, gets only a brief mention. The death toll is truly horrifying: towns are sacked, defenders hanged or beheaded en masse, bodies racked and roasted, traitors tied in sacks and drowned. The other option, being thrown into an oubliette or kept in chains on bread and water for life, doesn't sound any more attractive.

The reader puts down this exhausting narrative with one question: what was it all for? The ideals of chivalry were left in the mud at Agincourt and this book is inevitably darker in tone than its predecessor. Still, a baffling, tragic and wasteful episode has now been turned into military history of a high order. For England and Saint George!