On 25 October 1415 (Saint Crispin's Day), the English won an unexpected victory near Agincourt in Northern France against a numerically superior French army. The victory was decisive, decimating the political and military leadership of France, and started a new period of English dominance in the Hundred Years War which had begun in 1337 and was to last until 1453.

After several decades of relative peace, the English (under Henry V) had resumed the war in 1415 amid the failure of negotiations with the French. 

He claimed the title of King of France through his great-grandfather Edward III of England, although in practice the English kings were generally prepared to renounce this claim if the French would acknowledge the English claim on Aquitaine and other French lands.

He initially called a Great Council in the spring of 1414 to discuss going to war with France, but the lords insisted that he should negotiate further and moderate his claims.

However, the French mockingly responded to the English claims, and on 19 April 1415, Henry again asked the Great Council to sanction war with France, and this time they agreed.

Henry's army landed in northern France on 13 August 1415, carried by a vast fleet of 1500 ships. The army of about 12,000 men and up to 20,000 horses besieged the port of Harfleur.  

Although the town surrendered on 22 September, the siege took longer than expected. and the English army did not leave until 8 October.

The campaign season was coming to an end, and the English army had suffered many casualties through disease.

Rather than retire directly to England for the winter, with his costly expedition resulting in the capture of only one town, Henry decided to march most of his army (roughly 9,000) through Normandy to the port of Calais, the English stronghold in northern France.

During the siege, the French had raised an army which assembled around Rouen, and after Henry V marched to the north, the French moved to block them along the River Somme.

They were successful for a time, forcing Henry to move south, away from Calais, to find a ford. The English eventually crossed the Somme and resumed marching north.

Without a river obstacle to defend, the French were hesitant to force a battle. They shadowed Henry's army while calling on local nobles to join the army.

By 24 October, both armies faced each other for battle, but the French declined, hoping for the arrival of more troops.

The two armies spent the night of 24 October on open ground. The next day the French initiated negotiations as a delaying tactic, but Henry ordered his army to advance and to start a battle that, given the state of his army, he would have preferred to avoid, or to fight defensively.

The English had very little food, had marched 260 miles (420 km) in two and a half weeks, were suffering from sickness such as dysentery, and were greatly outnumbered by well-equipped French men-at-arms.

The French army blocked Henry's way to the safety of Calais, and delaying battle would only further weaken his tired army and allow more French troops to arrive.

Accounts of the battle describe the French engaging the English men-at-arms before being rushed from the sides by the longbowmen as the mêlée developed. 

Although the French initially pushed the English back, they became so closely packed that they were described as having trouble using their weapons properly. 

Recent heavy rain made the battle field very muddy, proving very tiring to walk through in full plate armour.

The deep, soft mud particularly favoured the English force because, once knocked to the ground, the heavily armoured French knights had a hard time getting back up to fight in the mêlée and some knights, encumbered by their armour, actually drowned in their helmets.

The French cavalry, despite being disorganised and not at full numbers, charged towards the longbowmen. It was a disastrous attempt. The French knights were unable to outflank the longbowmen (because of the encroaching woodland) and unable to charge through the array of sharpened stakes that protected the archers.

The plate armour of the French men-at-arms allowed them to close the 1,000 yards or so to the English lines while being under a terrifying hail of arrow shot.

A complete coat of plate was considered such good protection that shields were generally not used, however, in order to protect themselves as much as possible from the arrows, the French had to lower their visors and bend their helmeted heads to avoid being shot in the face, as the eye- and air-holes in their helmets were among the weakest points in the armour.

This head-lowered position restricted their breathing and their vision. Then they had to walk a few hundred yards (metres) through thick mud and a press of comrades while wearing armour weighing 50–60 pounds (23–27 kg), gathering sticky clay all the way. Increasingly, they had to walk around or over fallen comrades.

The surviving French men-at-arms reached the front of the English line and pushed it back, with the longbowmen on the flanks continuing to shoot at point-blank range. When the archers ran out of arrows, they dropped their bows and using hatchets, swords and the mallets they had used to drive their stakes in, attacked the now disordered, fatigued and wounded French men-at-arms massed in front of them.

The French could not cope with the thousands of lightly armoured longbowmen assailants (who were much less hindered by the mud and weight of their armour) combined with the English men-at-arms.

The impact of thousands of arrows, combined with the slog in heavy armour through the mud, the heat and difficulty breathing in plate armour with the visor down, and the crush of their numbers meant the French men-at-arms could "scarcely lift their weapons" when they finally engaged the English line.

The exhausted French men-at-arms were unable to get up after being knocked to the ground by the English. As the mêlée developed, the French second line also joined the attack, but they too were swallowed up, with the narrow terrain meaning the extra numbers could not be used effectively.

It is likely that the French at the back of their deep formation would have been attempting to literally add their weight to the advance, without realising that they were hindering the ability of those at the front to manoeuvre and fight by pushing them into the English formation of lance-points.

After the initial wave, the French would have had to fight over and on the bodies of those who had fallen before them. In such a "press" of thousands of men, many could have suffocated in their armour, which was known to have happened in other battles.

The French men-at-arms were taken prisoner or killed in the thousands. The fighting lasted about three hours, but eventually the leaders of the second line were killed or captured, as those of the first line had been.

According to contemporary English accounts, Henry fought hand to hand. Upon hearing that his youngest brother Humphrey, Duke of Gloucester had been wounded in the groin, Henry took his household guard and stood over his brother, in the front rank of the fighting, until Humphrey could be dragged to safety. The king received an axe blow to the head, which knocked off a piece of the crown that formed part of his helmet.

At some point after the initial English victory, after the English had overcome the onslaught of the French men-at-arms and the weary English troops were eyeing the French rear-guard, Henry became alarmed that the French were regrouping for another attack.

Henry's fear was that the prisoners (who, in an unusual turn of events, actually outnumbered their captors) would realise their advantage in numbers, rearm themselves with the weapons strewn about the field and overwhelm the exhausted English forces. A slaughter of the French prisoners ensued, sparing only the highest ranked (presumably those most likely to fetch a large ransom under the chivalric system of warfare.)

It seems it was purely a decision of Henry, since the English knights found it contrary to chivalry, and contrary to their interests to kill valuable hostages for whom it was commonplace to ask ransom. However, Henry threatened to hang whoever did not obey his orders.

When the battle was over, it was obvious that the French had suffered a catastrophic defeat. In all, around 6,000 of their fighting men lay dead on the ground.

The list of casualties, one historian has noted, "read like a roll call of the military and political leaders of the past generation". Among them were 90–120 great lords and bannerets killed, including three dukes, nine counts and one viscount (who was also an archbishop.)

Of the great royal office holders, France lost its constable (Albret), an admiral, the Master of Crossbowmen (David de Rambures, dead along with three sons), Master of the Royal Household (Guichard Dauphin) and prévôt of the marshals.

According to the heralds, 3,069 knights and squires were killed, while at least 2,600 more corpses were found without coats of arms to identify them.

Entire noble families were wiped out in the male line, and in some regions an entire generation of landed nobility was annihilated. 

The bailiffs of nine major northern towns were killed, often along with their sons, relatives and supporters. The battle cut a great swathe through the natural leaders of French society.

Estimates of the number of prisoners vary between 700 and 2,200, amongst them the dukes of Orléans and Bourbon, the counts of Eu, Vendôme, Richemont (brother of the Duke of Brittany and stepbrother of Henry V) and Harcourt, and marshal Jean Le Maingre.

While numerous English sources give the English casualties in double figures, record evidence identifies at least 112 Englishmen killed in the fighting. These included the Duke of York, the young Earl of Suffolk and the Welsh esquire Dafydd ("Davy") Gam.

Although the victory had been militarily decisive, its impact was complex.

It did not lead to further English conquests immediately as Henry's priority was to return to England, which he did on 16 November, to be received in triumph in London on the 23rd. Henry returned a conquering hero, seen as blessed by God in the eyes of his subjects and European powers outside France.

It established the legitimacy of the Lancastrian monarchy and the future campaigns of Henry to pursue his "rights and privileges" in France.

Other benefits to the English were longer term.

Very quickly after the battle, the fragile truce between the Armagnac and Burgundian factions broke down. The brunt of the battle had fallen on the Armagnacs and it was they who suffered the majority of senior casualties and carried the blame for the defeat.

The Burgundians seized on the opportunity and within 10 days of the battle had mustered their armies and marched on Paris. 

This lack of unity in France allowed Henry eighteen months to prepare militarily and politically for a renewed campaign.

When that campaign took place, it was made easier by the damage done to the political and military structures of Normandy by the battle.

Although many folk songs were written about the battle, the most famous cultural depiction of the battle today is William Shakespeare's Henry V, written in 1599.

The play focuses on the pressures of kingship, the tensions between how a king should appear – chivalric, honest, and just – and how a king must sometimes act – Machiavellian and ruthless.

Shakespeare illustrates these tensions by depicting Henry's decision to kill some of the French prisoners, whilst attempting to justify it and distance himself from the event. This moment of the battle is portrayed both as a break with the traditions of chivalry and as key example of the paradox of kingship.