We think of armies as professional bodies, thoroughly trained, wearing uniforms, following discipline, able (in the US) to get their tuition paid to go to college. Since the era of the Vietnam War, the US has gone from a draft to a volunteer army.
Medieval armies were very different, starting with the lack of uniforms, training, and discipline. The Roman army had been more like what we think of as an army, young men recruited into a paid professional military force for 20 year hitches, marching together in disciplined phalanxes. Medieval generals had read about Roman armies (especially the treatise by Vegetius, "the Art of War"), but good luck having anything like that after the economic and social collapse of the Empire in the sixth century.
The Germanic armies that were commanded by early medieval kings and counts were foot-soldier armies, supposedly made up of all able-bodied free men. Some had originally been hired by the Romans as "barbarian legions," but by the time one gets to the seventh and eighth centuries there was no sense of anyone getting paid. They were expected to turn out and fight to defend their people. Their chief weapon was a long sword, and they carried round shields—meant to protect the individual in a fight, rather than the tall, rectangular shields of the Romans, with which one could form a shield wall. They had helmets but not much in the way of armor.
In the ninth and tenth centuries, once stirrups began to be in use (apparently they began in Persia), and there was enough iron to shoe horses, horses began to appear more and more in armies. Knights started appearing attached to all armies in the eleventh century, and by the twelfth century knights made up the bulk of most armies, although they always had a significant foot-soldier component. Great lords, especially in England, were expected to show up for battles accompanied by a certain number of knights. If you think it would have been hard keeping a foot-soldier army disciplined when they were just young men told they needed to come defend the county, think about a lot of proud knights who were intensely proud of their ability and touchy about their honor.
These armies were still formidable. Knights and the accompanying foot soldiers went on Crusade, conquering the Holy Land in the First Crusade and establishing the Kingdom of Jerusalem. They lost it again within three generations, but that was probably inevitable for an occupying army surrounded by an awful lot of people who didn't want them there.
Mercenaries appeared in local wars in the eleventh and twelfth centuries, soldiers who would (at least theoretically) do what they were ordered to do, because otherwise they wouldn't be paid. They could also (theoretically) be counted on for a long war, whereas knights usually went home from regional wars after a while. The problem with mercenaries of course is if you stop paying, they will stop fighting for you, even switch sides.
Twelfth- and thirteenth-century knights had chain mail, helmets, lances, swords, and either round or tall, kite-shaped shields (as seen below). Plate mail, such as you see in Hollywood movies, did not appear until the late Middle Ages.
A lot changed for armies in the Late Middle Ages. Once gun powder became a weapon of war in the fourteenth century, during the Hundred Years War between France and England, cavalry charges became much less effective, as they could be brought down by cannon fire. More and more armies were made up of foot soldiers armed with pikes. Archers, both longbow men and those armed with crossbows, continued to play a major role, because there was nothing like a personal firearm.
The mass of foot soldiers of a late medieval army were treated with supreme disdain by their commanders, who called them cannon fodder, as they might be ordered to march against a bank of cannons and get killed, so the better trained soldiers could rush in before the cannons were reloaded. These soldiers were "recruited" by officers going around to villages and ordering a certain number of young men to join the army. They would end up with the poor who didn't have the money to buy their way out, the obnoxious and violent who were pushed to go by their neighbors, and the foolish, who actually believed the promises of military pay.
Yet these armies were tough. For the Hundred Years War, from which we have fairly good records, soldiers would march 20 miles in a day and then fight a battle.
© C. Dale Brittain 2021
For more on medieval knights and so much more, see my ebook, Positively Medieval: Life and Society in the Middle Ages. Also available in paperback.