The Battle of the Golden Spurs

This battle was fought between the French crown and the guilds and city-states of Flanders on July 11, 1302 in Kortrijk.

from Wikipedia:

The reason for the battle was a French attempt to subdue the county of Flanders, which was formally part of the French kingdom and added to the crown lands in 1297, but resisted centralist French policies. In 1300, the French king Philip IV appointed Jacques de Châtillon as governor of Flanders and took the Count of Flanders, Guy of Dampierre, hostage. This instigated considerable unrest among the influential Flemish urban guilds.
After being exiled from their homes by French troops, the citizens of Bruges went back to their own city and murdered every Frenchman they could find there on May 18, 1302, known as the Brugse Metten. According to legend, they identified the French by asking them to pronounce a Dutch phrase, scilt ende vriend (shield and friend), and everyone who had a problem pronouncing this shibboleth was killed.

The French king could not let this go unpunished, so he sent a powerful force, led by Count Robert II of Artois. The Flemish response consisted of two groups; one group which consisted of 3,000 men from the city militia of Bruges, was led by William of Jülich, grandson of Count Guy, and Pieter de Coninck, one of the leaders of the uprising in Bruges. The other group, which consisted of about 2,500 men from the suburbs of Bruges and the coastal areas, was headed by Guy of Namur, son of Count Guy, with the two sons of Guy of Dampierre; the two groups met near Courtrai. From the East came another 2,500 men, led by Jan Borluut from Ghent, and yet another 1,000 men from Ypres, led by Jan van Renesse from Zeeland.
The Flemish were primarily town militia who were well equipped, with such weapons as the Goedendag and a long spear known as the Geldon. They were also well organized; the urban militias of the time prided themselves on their regular training and preparation, which allowed them to use the Geldon effectively. They numbered about 9,000, including 400 nobles. The biggest difference from the French and other feudal armies was that the Flemish force consisted solely of infantry.
The French were by contrast a classic feudal army made up of a core of 2,500 noble cavalry, including knights and squires. They were supported by 1,000 crossbowmen, 1,000 spearmen and up to 3,500 other light infantry, totaling around 8,000.[2] Contemporary military theory valued each knight as equal to roughly ten infantry.

After the Flemish unsuccessfully tried to take Kortrijk on July 9 and July 10, the two forces clashed on 11 July in an open field near the city.
The layout of the field, crossed by numerous ditches and streams, made it difficult for the French cavalry to charge the Flemish lines. They sent servants to place wood in the streams but did not wait for this to be done. The large French infantry force led the initial attack, which went well, but French commander Count Robert II of Artois recalled them so that the noble cavalry could claim the victory. Hindered by their own infantry and the tactically sound position of the Flemish militia, the French cavalry were an easy target for the heavily-armed Flemish. When they realized the battle was lost, the surviving French fled, only to be pursued over 10 km (6.2 mi) by the Flemish.
Prior to the battle, the Flemish militia had either been ordered to take no prisoners or did not understand (or care for) the military custom of asking for a ransom for captured knights or nobles;[1] modern theory is that there was a clear order that forbade them to take prisoners as long as the battle was as yet undecided (this was to avoid the possibility of their ranks being broken when the Flemish infantry brought their hostages behind the Flemish lines).[4] Robert of Artois was surrounded and killed on the field.

The large numbers of golden spurs that were collected from the French knights gave the battle its name[5]; at least a thousand noble cavaliers were killed, some contemporary accounts placing the total casualties at over ten thousand dead and wounded. The French spurs were hung in the Church of Our Lady in Kortrijk to commemorate the victory, and were taken back by the French eighty years later after the Battle of Westrozebeke.
Some of the notable casualties:
Robert II, Count of Artois, the French commander
Raoul II of Clermont, Lord of Nesle, Constable of France
Guy I of Clermont, Lord of Breteuil, Marshal of France
Simon de Melun, Lord of La Loupe and Marcheville, Marshal of France
John I of Ponthieu, Count of Aumale
John of Trie, Count of Dammartin
John II of Brienne, Count of Eu
John d'Avesnes, Count of Ostrevant
Godfrey of Brabant, Lord of Aarschot
Jacques de Châtillon, Lord of Leuze
Pierre de Flotte, Chief Advisor to Philip IV the Fair.

The battle was one in a string during the 14th century that showed that knights could be defeated by disciplined and well-equipped infantry (one example is the Battle of Sempach in 1386). The Scots then applied this idea of attacking infantry and brought it to the battlefield at Bannockburn, where the Scottish Schiltron charged English Cavalry and routed them. It is also a landmark in the development of Flemish political independence and the day is remembered every year in Flanders as the Flemish Community's official holiday.
The battle was romanticised in 1838 by Flemish writer Hendrik Conscience in his book The Lion of Flanders (Dutch: "De leeuw van Vlaanderen"). Another unusual feature of this battle is that it is often cited as one of the few successful uprisings of peasants and townsmen, given that at the time most peasant uprisings in Europe were quelled.
“
The uprising originated from the people themselves, without being provoked by a lord (the Flemish count and his most important lords were in French captivity). Only when the uprising became widespread, the count's relatives who still were free rushed in to aid. But in the first place this was a struggle of people against a lord (the French king), not the struggle between two lords.”
Barbara Tuchman describes this as a peasant uprising in A Distant Mirror. Though the winning army was well armed, the initial uprising was nonetheless a folk uprising. Eventually, however, the Flemish nobles did take their part in the battle—each of the Flemish leaders were of the nobility or descended from nobility, and some 400 of noble blood did fight on the Flemish side.
The outcome of the battle, the fact that a large cavalry force, thought invincible, had been annihilated by - relatively - modest but well-armed and tactically intelligent infantry was a shock to all military leaders in Europe. It resulted in the end of the perceived supremacy of cavalry and led to a deep re-thinking of military strategies.
The slaughter of about 40% of French noblemen led to a deep crisis in French nobility but strengthened the king's position.