Henry V, King of England, had set his sights on conquering France ever since he came of age. When his father, Henry IV, came to the throne, the first English king to make his coronation oath in English, not French, there was little left of England's once vast holdings across the Channel, acquired through conquest and marriage portion. Even Normandy, which came to England with William in 1066, was lost, but the ambition to conquer and recover remained.

Although his father may have usurped the throne from Richard II, Henry V had no misgivings about his right to rule England and set out to rule France as well. After many attempts at negotiation to gain more of France had failed, in 1415 a tempting situation presented itself to the warrior king. France’s Charles VI was subject to fits of insanity and unable to rule. His brother, the able and loyal Louis d’Orleans, defended the king against their youngest royal cousin, John-the-Fearless, Duke of Burgundy, who coveted the throne. Skirmishes between the Orleanists, the king’s party, and the Burgundians for their duke, did not involve the whole country, but they disrupted farming, harvests and commerce generally. When the kings’ two eldest sons, both married to Burgundian princesses, died suddenly of poison, no one was in any doubt about the ambitions of John-the-Fearless.

Watching the constant civil unrest in France, Henry V persuaded parliament to grant him the funds to invade. The wiser of the French king’s counsellors begged the two royal factions to put aside their quarrels, and join forces. Alone, neither party could defeat the English king, clearly preparing an invasion. In August 1415, he came, landing his army at Harfleur. It took Henry far longer to subdue the port than anticipated, and it was late September before he left, a garrison holding what he had won. With his army reduced due to the fighting, the disease every commander dreaded – dysentery – spread through the English forces reducing them further.

With winter approaching, Henry needed reinforcements or he risked losing Harfleur, all he had gained and not enough to convince parliament to vote him more money for a second invasion. He must stay in France, march north and await re-inforcements in the spring, but first the army must cross the wide Somme with its steep sides. Finally, a fording place was found but far south and then the long march north again in never-ending rain. Spies informed him of the gathering French army, united for once, and determined to rid their country of the English “goddons”.

As well as their bows and arrows, the king’s 4,000 longbow men carried their sharp-ended stakes to plant in front of their positions to deter the enemy cavalry. His infantry marched carrying all they could, staggering in the omnipresent mud. It had rained for a week when Henry’s sick and tired troops reached the top of an incline, not much of a hill, with a ruin, a little shelter, where they gratefully bivouacked. Locals called it Agincourt.

In the morning, at last the rain had ceased. It was when the weak autumn sun rose and dispersed the mist shrouding the valley below, that the soldiers’ hearts sank. Below them, emerging from the shadow of the valley, on either side of their line of vision, they began to make out a huge, shining mass of movement, cavalry cavorting, knights in fine armour, a sea of soldiers, their myriad coloured banners dancing in the breeze. So many more of them than of us they whispered; and so close they could almost see the glow of triumph on the faces of the young French knights as they prepared to do battle, for many their first; sure of victory, even wearing parade armour, fancy but not practical. They had begged to be in the forefront, to win their spurs, be there at the moment of victory.

Henry watched too – not only the enemy below, but the spirits of his men sink as they gazed down at their enemy already wearing their pre-emptive glory. Leadership was personal; and he rallied them with valiant words on this Saint Crispin’s Day, urging his four thousand archers to blot out the sun, turning bright day into night with the density of their arrows. They could fire six a minute; accurate enough to pin a man to a saddle through his thigh, their barbed arrow heads tearing flesh when removed.

Just then, the French charged at the enemy on the hill, not on the orders of their commander in chief, the Constable of France, but of their individual dukes. The Constable knew the effect the weight of the heavy armoured horses and men would have on the field after the constant rain, but his shouts to hold them back were blown away in the roar of the advancing cavalry. It did not take long, not many strides for the heavy armoured horses and riders to be mired deep in bog, unable to advance, unable to retreat with more advancing from behind. Those who dismounted stood knee-deep in mud, flaying, helpless.

And the English? Clever young infantrymen took off their shoes – bare feet slip in and out of mud more easily – and ran down the slope towards the French stuck on their mounts or standing in the mud. Those they did not take prisoner, were dispatched with their short stabbing swords, plunging them deep into the hearts of the scions of France.

Henry V, fighting in the midst of the battle, saw he had not enough men to guard the captives, their ransom his soldiers’ prize, he issued the chilling order: No prisoners! And all had their throats slit. On this day chivalry died.

Accounts vary but it is estimated that some fifteen thousand French lay dead in the mud at Agincourt, and about two hundred English. And incredible victory for Henry V with devastating results for France.

Princess Michael of Kent’s new novel Quicksilver (published by Little, Brown) solves a 550-year-old murder mystery and completes her Anjou Trilogy of historical novels