Military Aspect of Medieval Europe
To understand why Medieval Europe was ruled by nobles and defended by mounted and heavily armed knights, you need only look at two changes wrought by the German tribes replaceing the Roman empire . The Romans had long run their lands in a very modern fashion. Form nearly a thousand years, the Roman Senate, as well as local legislative bodies, passed, and enforced, laws dealing with taxation and the establishment of an army, a system little modified with the introduction of the imperial system around the time of Christ. The taxes were collected, and the money used to pay for an army of Roman citizens. No other armies were allowed, and for a nearly thousand years, the Roman citizen armiess kept the barbarians out of Roman territory. In ancient times, the principal function of a government was providing for the security of the people, and for providing courts to settle disputes between citizens. Courts were cheap, armies were expensive. The Romans managed to provide a taxation system that provided the 10-20 million ducats a year it required to maintain a Legion of 5,000 trained troops. The Romans even provided retirement benefits for those troops who survived their 20-25 years of service. Through most of the Roman period, it required only 20-30 Legions (each roughly equivalent to the modern infantry division) to keep the empire secure, supported by a roughly equal number of auxiliary troops. This was a notable feat, for the empire at its peak had 70-100 million people, all being well protected by something like 300,000 troops. Few nations today, of similar size and population, get by with such a relatively small army.
Unfortunately, the Roman civilian government gradually became ineffective over the centuries, and so did the Roman army. The barbarians moved in and Roman administration and professional soliders were seen no more. Gradually, a new system, based on German tribal practice, grew up to replace it. Unlike the Romans, who developed out of a city state and its citizen militia, the Germans were a tribal people with a different form of militia. In the German system, exceptionally effective warriors were recognized and these warlords organized small groups of followers. The best of these warlords became the war leader in times of need. The Germans soon adopted the Latin term Dux (leader) for this fellow. Later this became a title of nobility, as in "Duke" (or "Duc" in France). In England, the Germanic title Earl (roughly equal to a Count) came into use, one of the few without a Roman root. Charlemagne, and other early Medieval kings, developed a system whereby the warlords were given sufficient land and serfs to provide an income to support themselves. These warlords became what we know of as knights. In return for land, and the people to work it, the knights were obligated to remain always ready for war and to answer the king's summons to go forth and campaign. Each knight was actually the leader of a small band of mounted and foot warriors. The number of troops each knight had to supply depended on how much land the king had granted, or any other special contractual arrangements that were made. The deal was not intially hereditiary, but gradually became so. If the knight's family died out (no male heir capable of bearing arms and leading his own troops), the land went back to the king (who then gave it to someone who could raise the troops.) The area the knight was given came to be called a fief, a word deriving from an olf term for "oath," since these obligations were sealed by oaths. Each province, or collection of scores or hundreds of fiefs, was governed by a Comites (another Latin title, for a provincial official). This title came to be known as "Count" (or "Comte" in France). The count was also a warlord and was responsible for leading his own personal troops as well as all of the other knights and warriors in his province. Like the knight, the count's family held the lands and powers for as long as they could produce the warriors to lead the troops and administer the province.
There were still taxes, but not as efficiently levied or collected as by the Romans. This changed over time, but meanwhile the money to pay for troops came from whatever the fief holders could squeeze out of their lands, more often in.kind than in cash. The fiefs employed what came to be known as the Manor System. Because the fief holder was also the local keeper of the king's peace, he sat in judgements of local legal disputes. He was the law, although local practice might call for a jury in some cases. Not all nobles were granted the power to pass a death sentence, but their authority in legal matters was still substantial. The fief holder had a large array of powers to collect fees on his lands. Of course, the inhabitants of a fief also had their own complex collection of rights and responsibilities. Some farmers were serfs, one step above slavery. Serfs could not move from the land they worked, had to give a large portion of their produce to the fief holder, and were obliged to work on the fief holder's private lands a certain number of days each month. They were better off than slaes in that they could not be sold, and had legally binding rights as a Christian. At the other extreme there were freemen who owned their own land. They were still under the local fief holder's legal authority, but were usually seen by the fief holder as an ally, someone who was nearly, but not quite, their equal. Being a fiefholder was hard work, since one had to supervise the estates, tend to local legal business, and keep in readiness for war.
As a result, even before the 14th century, fewer and fewer fief holders bothered to become knights. At the start of the Hundred Years War, there were only a thousand knights in all of England. But the English kings, in particular, had encouraged this. They preferrred that the fief holders stick with their administrative tasks and pay taxes in lieu of military service. The king then hired mercenary soliders when he needed them. The hundred or so great nobles in England still maintained their military skills and the king used these men as the senior officers in his army. By the 15th century, France had also adopted the English system of more efficient taxation and professional armies. It still wasn't as efficient as the Roman system, that would come in the 16th and 17th century when standing military units appeared, composed of long term professionals in the king's pay. In the next few centuries, it all came full circile, with national legislatures passing laws to raise taxes to maintain standing armies of professional troops raised from the citizenry. It had taken over a thousand years, but the professional armies eventually returned to Western Europe. Of course, the eastern Romans (Byzantines) had kept the taxatin/professional army system going until nearly 1453, but just barely and with a lot of feudal modifications..