Recruiting
In the earliest Middle Ages it was the obligation of every noble to respond to the call to battle with his own equipment, archers, and infantry. This decentralized system was necessary due to the social order of the time, but could lead to motley forces with variable training, equipment and abilities. The more resources the noble had access to, the better his troops would typically be. Typically the feudal armies consisted of a core of highly skilled knights and their household troops, mercenaries hired for the time of the campaign and feudal levies fulfilling their feudal obligations, who usually were little more than rabble. They could, however, be efficient in disadvantageous terrain. Towns and cities could also field militias.
As central governments grew in power, a return to the citizen and mercenary armies of the classical period also began, as central levies of the peasantry began to be the central recruiting tool. It was estimated that the best infantrymen came from the younger sons of free land-owning yeomen, such as the English archers and Swiss pikemen. England was one of the most centralized states in the Late Middle Ages, and the armies that fought the Hundred Years' War were mostly paid professionals. In theory, every Englishman had an obligation to serve for forty days. Forty days was not long enough for a campaign, especially one on the continent. Thus the scutage was introduced, whereby most Englishmen paid to escape their service and this money was used to create a permanent army. However, almost all high medieval armies in Europe were composed of a great deal of paid core troops, and there was a large mercenary market in Europe from at least the early 12th century.
As the Middle Ages progressed in Italy, Italian cities began to rely mostly on mercenaries to do their fighting rather than the militias that had dominated the early and high medieval period in this region. These would be groups of career soldiers who would be paid a set rate. Mercenaries tended to be effective soldiers, especially in combination with standing forces, but in Italy they came to dominate the armies of the city states. This made them problematic; while at war they were considerably more reliable than a standing army, at peacetime they proved a risk to the state itself like the Praetorian Guard had once been. Mercenary-on-mercenary warfare in Italy led to relatively bloodless campaigns which relied as much on manoeuvre as on battles, since the condottieri recognized it was more efficient to attack the enemy's ability to wage war rather than his battle forces, discovering the concept of indirect warfare 500 years before Sir Basil Liddell Hart, and attempting to attack the enemy supply lines, his economy and his ability to wage war rather than risking an open battle, and manoeuvre him into a position where risking a battle would have been suicidial. Macchiavelli misunderstood the indirect approach as cowardice.
The knights were drawn to battle by feudal and social obligation, and also by the prospect of profit and advancement. Those who performed well were likely to increase their landholdings and advance in the social hierarchy. The prospect of significant income from pillage and ransoming prisoners was also important. For the mounted knight Medieval Warfare could be a relatively low risk affair. Nobles avoided killing each other, rather preferring capturing them alive, for several reasons—for one thing, many were related to each other, had fought alongside one another, and they were all (more or less) members of the same elite culture; for another, a noble's ransom could be very high, and indeed some made a living by capturing and ransoming nobles in battle. Even peasants, who did not share the bonds of kinship and culture, would often avoid killing a nobleman, valuing the high ransom that a live capture could bring, as well as the valuable horse, armour and equipment that came with him. However, this is by no means a rule of medieval warfare. It was quite common, even at the height of "chivalric" warfare, for the knights to suffer heavy casualties during battles.