During the 10th century, two major changes came to the land of the Franks. They were unconnected, but each contributed in its own way to the establishment of medieval feudalism.
The first monastery in France was in Tours, established by St. Martin in the 5th century. Monasticism was not a widespread movement at first, but by the time of the Carolingians, the kings were donating land to build some early Benedictine houses. Although there were other monastic traditions, the Order of St. Benedict was the only option in Rome’s region of influence.
By the 900s, there were many more Benedictine monasteries. This was all well and good, but it created an inheritance problem. In the Frankish Salic Law, the landowner could divide as he chose, in theory among his sons, but actually among anyone. Many landowners willed away large sections of their land to the local monastery in exchange for perpetual prayers. They left some land to their sons, but it was diminished in size overall; and each son received an estate that was far inferior to his father’s.
Unfortunately, these pious dying Franks had been part of a Military-Agricultural Complex. The Carolingian dynasty depended on each subdivision of land to support horses and armed men. It was how the army was raised; each lord was responsible for mustering (and training) a certain number. But now, estates were being whittled away from within by non-taxable church ownership. After a few generations of piety, there might be barely enough land left to support the barony’s required horse-breeding, iron-smelting, and weapons-training.
In addition to the attacks of Saracens in the south and Slavs in the east, the Franks now suffered attacks of Danes in the north. Although they had similar cultures, the Franks had become the fat, soft-bodied targets of their lean, hungry northern relatives. At one point, the Danes sailed up the Seine River and burnt Paris.
Charles the Simple, King of West Frankia and Lotharingia (Lorraine), solved the Danish problem. In 911, he made a treaty with Rollo (Danish Rolf), leader of the most threatening band of Danes. Rollo and his men were given a large territory along the Atlantic coast, north of Aquitaine. In exchange, they became part of the homeland defense against other Danes. The “Land of the Northmen” was shortened to Normandy.
I think that the Normans came mostly as single males and married local wives, who raised the children as Frankish-Latin speakers, because their Danish language remained only in names (especially men’s names). Within two generations, Normans all spoke French. In some ways, they stayed culturally distinct, like the Gauls of Provence. They were even more lightly converted to their new Christian religion than the Franks, and they remained much more warlike for centuries. They also kept their Norse inheritance law of primogeniture: total inheritance by the oldest son.
By the 11th century, the French kings forced inheritance reform. And as among the Normans, the rest of a man’s land had to pass to his oldest male heir in one piece. It was unfair to the younger sons, but it was much better for the people who lived under them. The law also put a top limit on how much land could be donated to monasteries.
Alliances became more stable and boundaries stopped changing with every generation. Primogeniture became the first line of defense in the ongoing war against Eastern invaders. In later times, social reformers pointed out how primogeniture kept all the wealth in the hands of a few. The Carolingian kings might rejoin: “You say that like it’s a bad thing.”