Perfectly timed for the 600th anniversary of the Battle of Agincourt, the fourth volume of Jonathan Sumption’s epic narrative of the Hundred Years’ War takes the story from Richard II’s death in 1399 to Henry V’s in 1422. The period, as Sumption lucidly explains, is marked by two brutal political assassinations. In 1407, Louis, Duke of Orléans, effectively ruler of France while his brother King Charles VI, a paranoid schizophrenic, was ‘absent’ (as the common euphemism had it), was bludgeoned to death by assailants hired by the venal and unscrupulous John the Fearless, Duke of Burgundy. Twelve years later, John was himself scythed down on the bridge of Montereau-Fault-Yonne, a fortress on a spur of rock at the confluence of the Seine and the Yonne. Although shown by Sumption to be acting with the explicit knowledge and consent of the French dauphin, Charles, John’s murderers (or most of them) had once been the protégés of Orléans: they thirsted for revenge. A bloody civil war in France, triggered by the first of these murders and then intensified by the second, gave Henry IV, King Richard’s supplanter, and afterwards his charismatic, energetic, imperious son Henry V their cues. As Francis I would be tartly informed when he made an unscheduled detour in 1521 to view John the Fearless’s broken skull in his tomb at Dijon, ‘By that hole, the English entered France.’

Henry IV, the founder of a new dynasty, adopted a cautious strategy on the Continent – not unlike Henry VII just under a century later. Racked with morbid guilt over the manner of his coming to the crown and increasingly ill with a form of psoriasis that contemporaries mistook for ‘leprosy’ and a mark of sin, the first Lancastrian king lacked his son’s overpowering desire to emulate the glorious victories of Edward III and the Black Prince. Only after much prevarication would he intervene in the French civil war, and then merely to sell his services. Despite all his youthful pretensions, Henry IV was unwilling to live out the martial ideals assigned to the English monarchy by its more vocal propagandists.

While still Prince of Wales, the future Henry V championed the idea of a heroic role in war for the king. In Sumption’s superbly crafted, exhaustively researched, supremely intelligent narrative, we are provided with a long-awaited counterpoise to the endless round of lightweight popular histories and to the tenacious, if compellingly persuasive fictions of William Shakespeare. Like his chief Tudor votary, Henry VIII, Henry V was eager to win territory, plunder and immortal fame through his French campaigns. However, he was just as determined to establish the durability of his dynasty. What mattered most to him was his right to rule in the eyes of God and being acclaimed as a ‘king of justice’.

In the summer of 1415, Henry V landed just north of the Seine estuary at the head of the largest expeditionary army that the English had ever taken to France, including the biggest, most disciplined contingent of archers so far seen anywhere in Europe. Henry’s muscle was reinforced by twelve ‘great cannons’ or ‘bombards’. After a protracted siege, he took Harfleur, France’s principal northwest port. From there, according to his chaplain and one of his Gascon clerks, his aim had been to capture the coastal region of Normandy as far as Dieppe before advancing up the Seine valley towards Rouen and Paris.

Stricken by heavy losses owing not least to outbreaks of dysentery in the ranks, Henry ignored his councillors’ advice to return home directly and decided instead on a show of strength. It was towards the end of a 150-mile march through Normandy and Picardy towards Calais, the English garrison town surrendered to Edward III in 1347, that he was forced to fight a set-piece battle at Agincourt. Threatened with encirclement, bivouacked in the open on sodden ground where his forces were heavily outnumbered by more than eight to one in cavalry and two to one overall, Henry positioned his men carefully and kept a cool head, responding flexibly to events as they occurred, unlike the French, whose commanders were vacillating, quarrelsome and robotic in their attitudes.

The legend turns out to be true that some seven hundred prisoners were massacred after Henry’s great victory, but no one, surprisingly, held it against the English at the time. From Sumption’s visceral descriptions, far more threatening to contemporary social norms were successive eruptions of mob violence in Paris, typically involving butchers. In the summer of 1418, the urban proletariat found its head on a scale reminiscent of the Peasants’ Revolt of 1381 in England. It seems that time and time again – and in the countryside too – the struggle between the Burgundians and their opponents in the French civil war unleashed atrocities as barbarous as anything experienced during the 16th-century Wars of Religion in France or the Netherlands.

Between 1417 and the Treaty of Troyes in 1420, Henry V had to deal with his third dauphin in fifteen months. Charles, Count of Ponthieu, the eleventh of Charles VI’s twelve children, was an altogether tougher, more slippery individual than his dead brothers, a teenager with spindly legs and a narcissistic urge. Capturing the Cotentin peninsula in early 1418 and finally taking Rouen, and much of the rest of Normandy with it, in January 1419 after a desperate winter siege, Henry was to lead his armies to within twenty miles of a depopulated, partially devastated Paris. With an iron curtain fast descending between Burgundian and Dauphinist France and many Frenchmen believing that a deal with the English would be the lesser of two evils, Henry suddenly realised that his ambition to capture the French crown was realistic, rather than chiefly a bargaining counter. By the terms agreed at Troyes, he was married to Catherine of Valois, Charles VI’s daughter. He also became regent of France while Charles VI was alive and was to become king after Charles’s death. He entered what was left of Paris in triumph. In London, Catherine was as rapturously received as Catherine of Aragon would be some eighty years later.

Amid all the twists and turns of war and diplomacy, Sumption’s wonderfully measured prose is judiciously spiced by sharply observed, poignant or amusing snapshots. Deliciously, he informs us that to stamp his purportedly reforming agenda on the memories of his uncles and cousins early in the civil war, John the Fearless doled out New Year’s gifts, including jewelled masons’ levels and plumb lines, to symbolise his plans to straighten out the crooked ways of the royal administration. Describing the habits of the English as negotiators, he remarks that they tended to present themselves ‘like lawyers appealing to authority rather than politicians trading for advantage’. They would turn up clutching ‘beautiful and important-looking volumes’, which they would ostentatiously consult at frequent intervals. Acting as a spokesman in Paris in 1414, the urbane bishop of Norwich, speaking in Latin as a prelude to reading out an entire beadroll of preposterous demands, began in the fashion of a sermon with a quotation from the Book of Joshua: ‘We be come from a far country, now therefore make ye a league with us.’

In 1422, a fatal combination of a virulent bout of dysentery at Vincennes, on the outskirts of Paris, and the inevitable French misgivings at the terms they had conceded two years before put paid forever to Henry V’s dreams. Charles VI died just two months later. Would his funeral be the final public ceremony of the Valois monarchy, or would his eldest surviving son, Charles, from his strongholds south of the Loire conjure up the will – and the support – to expel the English from France? For the retelling of this final, arguably most piquant act of the drama, we keenly await Sumption’s fifth and concluding volume.