This change began almost immediately at the dawn of the High Middle Ages, but by the end of the 15th century the knight had all but disappeared as a decisive force at war.
The Crossbow
This weapon, which had been developed by the Chinese as early as 600 BCE, gave an infantryman the ability to kill an armored opponent from a distance, after very little training. This weapon was scorned by the knights, who saw it as unsporting because a man who had been training for knighthood since birth could be killed by a common man who had trained for a matter of weeks. In 1139 Pope Innocent II issued a ban upon them. Even the Magna Carta condemns them; in article 51 it states: “ As soon as peace is restored, we will banish from the kingdom all foreign born knights, crossbow men, sergeants and mercenary soldiers who have come with horses and arms to the kingdom's hurt." Neither of these measures stopped their widespread use, however.
The English Longbow
The next important weapon to be perfected was that which gave the English their fame during the Hundred Years' War against France: The longbow. This weapon was originally a hunting weapon imported from Wales, and it gave the English Yeoman the ability to fire an astonishing number of arrows at his opponents. With the development of the bodkin arrow, a long arrow with a heavy, pointed iron tip, the English longbowman could kill his armored opponents from hundreds of yards away. At the battle of Crecy in 1346, hundreds of thousands of arrows were fired in a matter of minutes, and the French cavalry charge was halted. The longbowmen also outperformed even the Genoese crossbowmen they faced at Crecy, who fired more slowly and at far shorter ranges.
The drawback to the Longbow was that it took years of training—at the related cost—and immense physical strength to use effectively, as the draw weight of the weapon was often as high as 160 pounds.
The Pikeman
Pike tactics, an ancient method of fighting pioneered by Alexander the Great's Macedonian phalanxes, gave the infantryman great staying power against cavalry. Essentially a long spear, pikes were pointed forward by men standing shoulder-to-shoulder. The spears which protruded from the rear ranks created a wall of spear points into which horses would refuse to charge. The Scots were pioneers of these tactics, using oval-shaped formations called schiltrons which had no flanks or rear, and thus no weak points for the cavalry to exploit.
The Flemish forces at the battle of Courtrai in 1302 were largely infantry armed with long spears and large clubs called Goedendags. They were tough and well-organized, and handily defeated the disorganized French army, killing over 1,000 knights.
It was the Swiss, however, who perfected the use of the pike. They combined incredibly discipline with the tactic known as the pike square, in which 100 men arrayed themselves facing to four sides of a square, and pointed their pikes outward. These squares were virtually impervious to a cavalry charge, and they were incredibly maneuverable, being able to turn and advance quickly and efficiently to meet any new threat. This made the squares nearly impossible to flank. It also took the primarily defensive nature of pike tactics and gave them a decidedly offensive twist. Using these tactics at the battle of Nancy in 1477, the Swiss defeated the Burgundian cavalry army of Charles the Bold and set the course of military thinking for the next few centuries, until the development of effective musketry.
Gunpowder
The death knell of the armored knight, indeed of the armored soldier in general, was the development of gunpowder. Using even a primitive gunpowder weapon, any form of personal armor could be rendered useless. As the weapons such as handguns and arquebuses developed (The forerunners of modern firearms), a line of soldiers wielding these weapons could cause frightful wounds to their opponents. They also frightened horses and men with the amount of smoke and noise which they produced..
The development of effective field artillery also ended the days of protracted sieges, as the stone walls of a castle could be blasted to bits with relative ease. This development was symbolic as well; It lessened the importance of the castle as the austere symbol of a lord's permanent authority over his lands.
Sources:
“All Things Medieval” at knight.medieval.com.
“Medieval Warfare” at hyw.com
“Pike Square” at http://www.lepg.org
“Medieval Crossbow” at castles.me.uk
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