One of the largest (by some estimates as many as a million people) and most successful of the German tribes were the Franks. They was actually a confederation of tribes which, tired of being dumped on by other Germans and by the Romans, came together in the 3rd century AD and adopted the common name "Frank" (derived either from the word "Free" or "Spear") In the 4th century, many of the Franks were living in the area of Belgium and the Rhineland as allies of Rome. But other Franks were living inside adjacent German territory. The Franks did little while the other German tribes came and went. Still, when the Goths in the east were conquered by the Huns, the Franks managed to stay independent, and lent a hand at defeatomh the Huns at the battle of Chalons.
When the Franks did move into Roman lands, they were more numerous than other German tribes and came to stay. Living alongside the Romans in Gaul for several generations, the Franks absorbed the lessons of Roman civilization and developed their own organized kingdoms. In the 5th century, with Roman authority gone, many of these Frankish kingdoms were united under Merovich (reigned 448-458). His grandson, Clovis (reigned 481-511), tidied up the frontiers a bit and also converted to Christianity. Thus organized and united, the Franks systematically conquered southern France and large parts of Germany. In most cases, they were conquering areas ruled by less numerous (and less well organized) German rulers. In some cases, autonomous Roman areas were conquered, as the destruction of Roman imperial power in the West had not prevented Roman power from remaining in force in some local areas, such as Brittany. So successful were the Franks in their nation building that all Germanic peoples became known to non-Europeans as "Franks." One part of the Frank nation, France, carried the original name into current times.
Throughout the 6th century, the Christian Frankish kingdom continued to develop. The Franks were keen to use Christianity, an educated clergy, and Roman methods to run their kingdom. But at the very top, the methods were German. The ancient German tribes had recognized clan leaders as descended from the gods, and thus fit to rule as a dynasty. This went against Roman practice, where the officials, and in theory even the Emperor, were selected from those judged most able to do the job. The newly Christian Germans took care of the "leader as related to the gods" angle by being "annointed" by a bishop. The Church developed the idea that the king was chosen by God (who was not considered an ancestor) to rule. The "Divine Right of Kings " was not a Christian invention but rather a Christian adaptation to an old German concept. The Romans had also used the same idea with their last bunch of (not very successful) pagan emperors.
The key concept of feudalism, the surrender of personal freedom in return for protection and material well being was a combination of German and Roman practice, and was most developed and most widely practiced by the Franks. The Roman feudal practices were older and had grown up in the frontier areas where refugees fleeing into Roman territory had to surrender freedom for protection. The Germans, entering Roman areas now stripped of central government and the Roman army, found defenseless people willing to accept the same terms. The Germans had long followed the practice of every able bodied adult male being trained and employed as a warrior. This idea had long since died out in Roman areas, where even recruiting soldiers for the regular army had become increasingly difficult. The Germans were as wolves among sheep in these Roman lands. It was a large scale protection racket, with the Germans, even after they blended into the population and lost their language and German customs, still forming the core of the feudal warrior class. This may explain why so many knights were blond and blue eyed.
A key element of feudal life was known as the "Manor System." This was nothing less than a company town largely owned and completely controlled by the local noble. This fellow might be a knight, or some higher ranking noble. The manor was was an area over which the noble ruled and supplied such vital services as justice . Some areas still had the Germanic jury system, others simply had the local lord holding court and being judge, jury and prosecutor. The lord often owned, or controlled, key services like milling grain, iron working or even baking bread. If the lord didn't own these facilities, he charged a fee for their use. The Germans found that the Romans had thoughtfully set up manors for them, in the form of large, self-contained farming communities owned by wealthy Romans. This was an ancient Roman practice, as it was considered the most efficient way to invest ones wealth. The farms were worked by slaves or tenant farmers, and the Germans took them over, turning the workers into serfs. The Germans had nothing against slavery, it was just that they, like the Romans, found that serfs (semi-free slaves, in effect) were more productive than slaves. The Franks, under a series of competant kings, turned the Roman estate practices into what became known as the Manor System .
By the 8th century, the heavily armed and increasingly armored Frankish warrior on horseback had demonstrated his superiority over any infantry force in Europe. It is likely that such cavalry forces would have fared poorly against Roman legions trained and disciplined in the classical manner. But the Romans had forgotten about training and lost their discipline before the Germans overran the empire and it would be another five centuries before an effective infantry army was again created (the English yeomen and the Swiss pikemen). Thus for five centuries, from the 700s to the 1200s, the man on horseback ruled the battlefield. But only if you had a lot of them.
These knights were expensive. The horse, arms and armor cost a minimum of 20,000 ducats, and usually a lot more. Annual repair and replacement cost several thousand ducats. Even more expensive was the training, about ten years worth, at some 5,000 ducats a year. Then, for the 10-30 years the knight was capable to fighting, supporting him would cost at least 5,000-10,000 ducats a year. But it got even more expensive than that because most knights married (the only exception being small number in religious military organizations). A wife and children were expensive, and knights were selected for their fighting, not their managemant skills. They tended to be spendthrifts and given to bad habits like gambling. An adult knight, with a wife and average spending habits, cost at least 30,000 ducats a year. Your typical knight could go through 100,000 ducats a year without any trouble, and not even think of himself as a spendthrift.
But it all came together in the 8th century, as the kings, especially the Frankish kings, realized that they could combine the manor system with all their knights to produce a system that would provide trained soliders for the crown and also put reliable men in local positions of authority throughout the kingdom. Some of the German nobles who adopted this system, especially the ones in former Roman lands, realized that it was basically an adoption of the 3rd century Roman system of giving the large estate owners government authority over their lands and people and the responsibility for raising troops. The western Romans never perfected this system, although the eastern Romans did. Their knights, called cataphracts, were probably the most effective in Europe. But there were never enough cataphracts to deal with all the Turks and Arabs closing in on eastern Roman lands.
Another uniquely German idea was the "fellowship ." This concept saw each German noble surrounded by lesser nobles and commoners bound to the big guy by a mutual bond of loyalty. The Romans had a similar arrangement with their system of clients and their noble (or simply rich) patrons. The German custom prevailed, as it was more warrior oriented. The Roman practice was used mainly in a political and economic setting. The Germanic idea of fellowship was that of a tight group of friends who would look out for each other in battle. Never forget that the Germans, until they settled down, were basically an army with families attached. War was the central element in their lives. They kept herds of animals, practiced primitive agriculture (which exhausted the soil and forced them to move) and hunted. But the favored form of economic activity was a succesasful war and the loot it would produce. Roman lands were always seen as rich, and when the Roman defenses declined, the Germans simply took advantage of an opportunity they had been waiting centuries for. Once they settled down on Roman land, these military fellowships became the basis for long term political relationships.
The Frankish kingdom culminated in Charles the Great (or Charlemagne, or Karl der Grosse). Since 628, Charlemagne's ancestrals had held the office of 'Mayor of the Palace,' a sort of Shogun to a series of increasingly feeble Merovingian kings, running the country in the name of the crown. Charlemagnes father, Pepin the Short (reigned 751-768) ended the line of the inept Merovingians by sending the last one to live out his life in a monastary. Given the usual customs of the time, this was an exceedingly civilized way to change dynasties. Pepin's son was Charlemagne (reigned 768-814), an all around competant fellow who made things happen. Charlemagne expanded and organized the kingdom. He spent most of his warm weather time leading his armies. His Winters were spent organizing the kingdom and encouraging scholarship. As a reward for helping the pope out against some Lombard enemies, in 800 Charlemagne was crowned "Roman Emperor," initiating the so-called "Holy Roman Empire. " Charlemagne's kingdom, at its height in the early 9th century, encompassed all of France, Switzerland, Belgium, and Holland, as well as most of Germany and parts of Spain, Czechoslovakia, Austria, and Italy. But that empire didn't last, no matter how well it was run. The German practice was to divide the father's holdings among his sons, and Charlemagne had the misfortune to have one son who himself had four sons. During the 9th and 10th centuries, Charlemagnes empire basically divided into two parts, German and French. The French half (most of present day France) was quite Romanized and most of the population spoke dialects of Latin . The German half of the spoke German, and still does. The 842 document that made this division official, the Oath of Stasbourg, was written in German and French in recognition of this division.
For a number of reasons, the French portion of the Frankish kingdom remained united while the German portion broke into a muddle of independent states that were not united until 1870. The major advantage the French speaking territory had was their Roman heritage, and the abandonment of inheritance by division. The area had been ruled by Roman administrators for centuries and the inhabitants looked back on that time as the Good Old Days. Even the new German --albeit increasingly Romanized-- overlords, who by now had been around for over two centuries, agreed. Everyone spoke French (or one of the many dialects) and a long string of able French kings proceeded to increase the unity of what they continued to call "The Frankish Kingdom" (ie, France). The Hundred Years War merely completed the process.