The battle of Agincourt, whose 600th anniversary falls on St Crispin’s Day, 25 October, is still tabloid gold, Gotcha! with chivalry. Henry V’s victory in the mud of Picardy remains the classic English away win and a victory snatched from the jaws of defeat. Agincourt is also a battle with a modern afterlife, whose story still provokes mixed reactions, especially among the French.

Celebrated immediately in popular song, its place in history is supported by several contemporary accounts, in French and English. Shakespeare had all the sources he needed to help conjure up “the vasty fields of France”.

The fighting took place on a strip of muddy ground sandwiched between two woods, decisive terrain. After the debilitating siege of Harfleur, immortalised in “Once more unto the breach, dear friends, once more …”, this was an engagement Henry V did not really want. His Welsh and English army was sick, starving and exhausted. Outnumbered on foreign soil, he faced an opponent who was fresh and ready for the fray. No wonder the king heard mass three times on the eve of battle.

Rarely has the fog of war been so impenetrable. Almost everything about the Anglo-French bloodbath at the place known as “Azincourt” is disputed, apart from the sensational outcome. Some say the French-English ratio was six to one. Recent scholarship puts the disparity at four to three. Most agree that Henry fielded perhaps 1,500 men-at-arms and about 6,000 archers.

According to French sources, anxious to explain a military catastrophe, Henry commanded a ruthless war machine. The night before the fighting began, it is said, he ordered his men to wait in the darkness in silence – on pain of losing an ear. Medieval warfare was medieval: the French vowed to cut off the archery fingers of every captured English bowman. On St Crispin’s Day itself, the French were well prepared, young and over-confident. Drawn up in three lines, or “battles”, they embodied shock and awe. After sunrise, for three hours, they simply held their position and played a waiting game, a strategy favoured by military textbooks.

The French had the better of this opening. As morning passed, Henry was forced into yet another gamble. He advanced his men, a risky manoeuvre which meant uprooting the defensive stakes behind which his archers were sheltering. The king’s counterintuitive move seems to have caught the French off-guard because, waiting no longer, the flower of their cavalry charged towards the English lines.

Famously, it was a disaster. A well-trainer archer could fire 10 arrows a minute. Under a murderous hail, the French cavalry was cut to pieces. Horribly injured horses, running amok, became a cruel part of the bloody mayhem at the clash of French and English forces. When more heavily armed French knights began to advance on foot, staggering through the mud, they were slaughtered at point-blank range, falling on the bodies of their comrades. When the archers ran out of arrows, they turned axes, swords and even mallets against the helpless mass of French chivalry wallowing in the mud.

Everyone agrees it was a massacre. Many French knights, trapped inside their expensive armour, suffocated or drowned in the mud.

The English breakthrough seems to have been comparatively swift. In his panicky order to execute the hundreds of French prisoners behind his lines, there is evidence that Henry could not quite comprehend the English victory. Overall, the death toll was appalling. French sources suggest that they lost between 4,000 and 10,000 men. Almost as bad, from the French point of view, its governing elite, including dukes and bishops, was annihilated.

Estimates of the English dead, by contrast, range from improbable (100) to plausible (1,500). Henry returned to London in triumph, parading through the city on 23 November. Already, as “Azincourt” became “Agincourt”, his victory was becoming anglicised. High and low culture responded to Agincourt in several important ways. At court, the English language was now the medium through which the king, an accomplished spin-doctor, promoted his success. Also in the vernacular, the Agincourt Carol, several ballads, and a hit play The Famous Victories of Henry V sealed the battle in the amber of folk-memory.

Henry continued to unify his people with aggressive nationalism – successive invasions of France. His death, from dysentery, in 1422, aged 35, cut short any imperial ambitions. He had, however, done something that had eluded his predecessors: he had avenged the battle of Hastings. The year 1415 should have been the definitive riposte to 1066, but the after-life of Agincourt is not straightforward. This is so for perhaps three reasons.

In the first place, not even Shakespeare could swallow the Agincourt Kool-Aid. In 1599, cheerfully plundering The Famous Victories of Henry V, he wrote Henry V, one of his most popular plays. On closer examination, however, Shakespeare is far from gung-ho. In a nervous opening, the play begins with a Chorus who entreats the audience to set aside scepticism and enter into the spirit of fighting the French. “Think, when we talk of horses, that you see them printing their proud hooves …” This edgy start is followed (shades of Iraq) by a long debate among some English clergymen about the legitimacy of an “offensive war”. Shakespeare’s Henry is portrayed as an inspirational king, but we also see him as an isolated leader, adopting disguise to mingle with his troops.

Second, there’s the enduring controversy (which Shakespeare does address) about the king’s cold-blooded execution of his French prisoners, a debate bedevilled by double standards: Henry must be judged by his mores, not ours. Nevertheless in 2010 a legal panel led by US supreme court judge Ruth Bader Ginsburg held a mock trial which found Henry guilty of war crimes. Shakespeare remained ambivalent. In Henry V the order to kill the prisoners is seen by his followers as just revenge for the murder of boys guarding the baggage train.

Facebook Twitter Pinterest Laurence Olivier in his film version of Henry V. Photograph: ITV/REX Shutterstock

Third, Agincourt suffers from being one-sided. In the renewal of a great myth, all parties must participate. 2015 is also the 200th anniversary of Waterloo, a defeat whose afterlife is as much celebrated in France as in Britain. Charlotte Georges-Picot, head of press relations at Les Invalides, the tomb of Napoleon, and home of French military history, says “we love everything about Napoleon. Whenever we do something here about him, it’s – bingo!” If Waterloo is box office, so is Louis XIV, the Sun King, and also General De Gaulle. A recent exhibition at the Musée de l’Armée explored wartime relations between Churchill and De Gaulle. “Of course,” says Georges-Picot, who understands what’s going on here, “we play up ‘the old enemy’. There’s no hostility any more. It’s just a bit of fun.”

And yet there’s something about what happened on that remote field in northern France on 25 October 1415 that lingers like a long-buried hurt. When it comes to Agincourt, the French, who generally idolise military matters, are either silent or ignorant. “This was just such a bad defeat,” says Georges-Picot. She adds that this humiliation is not taught in schools, where Henry V of England is unknown. It is almost as if neither the battle nor its victor had ever been. Almost, but not quite. In Paris next month the Musée de l’Armée will mount an exhibition that brings together some long-lost treasures from the 15th century.

Antoine Leduc, the youthful curator of Chevaliers and Bombards, who grew up in a village 13 miles from Azincourt, has always been fascinated by medieval arms and armour. He is an informal French patriot. At the Musée de l’Armée, that is virtually a job description. When he joined the museum, Leduc was required to enlist in the army. Now, he says with a smile, “I am a civilian.” But Agincourt stuck in his mind. Five years ago he proposed an exhibition to mark its 600th anniversary. He remembers his superiors asking their new recruit to remember French battles in a slightly more positive way. They said: “Azincourt was a bad defeat for France – but of course we also have Marignano.”

Marignano 1515! It is a date known to every French schoolchild, the historic moment when François I – “a great king”, interjects Leduc – won a thrilling victory for his people against … the Swiss. The Swiss? Is this really the stuff of national legend? Leduc looks a bit sheepish. “It’s complicated,” he says. “The battle was part of our conflict with the pope.”

So that’s all right then. And highly instructive, too. Curator Leduc has put together an enthralling historical display in the museum at Les Invalides: 100 years of French military history, beginning with the shame of Azincourt but culminating in Marignano.

Leduc has his own theories about why the French do not remember Agincourt. “First of all,” he says, “it was a defeat … People prefer to celebrate victory. Second: the latest scholarly programmes do not illuminate the Hundred Years War, and the Middle Ages are not well known in France. Yes, French people love history. That is true. But it’s mostly heritage. Very few people really know history.” He indicates a brilliant display of medieval armour. “That is why we do exhibitions.” Mobilising France is not quite what it used to be.

Knights and Bombards, Agincourt 1415 to Marignano 1515, is open at the Musée de l’Armée, Les Invalides, Paris, from 7 October until 24 January 2016