Even before the emergence of William of Normandy as William the Conqueror, the use of feudal armies began to diminish. This reason for the change in army forces is, for the most part, summed up in one word: Mercenary. Therefore, we must explore this phenomenon in order to trace the roots from feudal army to the armies-for-hire that became the norm throughout conquered, yet not always so-unified England.
It was already customary for every able-bodied male within England to serve in the armed forces, and contrary to the German practice of the same obligation, it often times could be enforced through use of pain or extreme penalties. This decree, although very seldom relied upon, became known as the fyrd. Because for so long it was unnecessary to call out the fyrd, it is obvious to expect that the quality of these 'peasant'-soldiers would be severely lacking in tactical knowledge and weaponry skills.
This lack of skilled warrior was apparently evident. As the incorporation of a "select fyrd" was soon implemented in order to fulfill the king's regular military mission and needs--and the recruitment of this select fyrd was almost parallel to the Carolingian practice of grouping four mansi in order to require one able and armed soldier per group. This eventually led to the thegns, these were soldiers that belonged to the personal entourage of the monarchs, and who served for a longer period of service. This longer service was due to their receiving a grant of land or for merely receiving the honor of a higher position in society--as was more often the case, as the royal thegns tended to live in the houses of their superiors.
Unfortunately, the small amounts of 'specialized' fyrd or royal thegns, could not suffice as a regular army, and even their skills were...