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 burned Paris.
Charles the Simple, King of West Frankia and Lotharingia (Lorraine), solved the Danish problem in his locale. In 911, he made a treaty with Rollo (Danish Rolf or Hrolfr), 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. It was not at first known as a duchy, but its early rulers were Counts.
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.
Normandy was not empty. Its inhabitants were Bretons, who continued to fight to gain back their land until they were finally pushed into Brittany, a rump of the old Celtic kingdom. In 942, the Bretons and Normans signed a peace treaty. During the same period, the Frankish lord Fulk was fighting both Bretons and Normans to create his own county of Anjou. Over time, Anjou’s lords intermarried with Normans, as did Blois, Champagne, and Brittany itself. The Norse-tinged Christian lords remained highly militarized for generations. This “Norman” culture powered the Crusades.
In 927, an aging Rollo was succeeded by his son William (in Norse, Vilhjalmr), who ruled until 942. William had dived right into the dynastic divisions of the Franks, supporting some factions against others. The Normans were thus quickly an indispensable part of Frankish politics. But they were not entirely Christianized by this time.
Rollo’s grandson Richard inherited power as a child. King Louis IV took the boy count to Paris, but a popular Norse uprising returned him to Normandy at age 14, where Count Richard chose to ally with the pagan Count of Bayeux and Norse King Harald Bluetooth. Count Hugh of Paris sought an alliance with Richard, betrothing him to his daughter Emma. In 955, even before he married Emma, Richard became the guardian of Hugh’s son Hugh. This second Hugh is known to history as Hugh Capet, the Count of Paris who was elected King of France.
Richard and Emma had no children, but Richard was now tightly allied to the King of France and determinedly Christian. His second marriage reached out to a rival tribe of pagan Norse around Cherbourg. Beautiful and intelligent, Gunnor gave Richard many children before he finally married her so that their second son could become an Archbishop. Oldest son Richard II became Duke of Normandy, while his sister Emma married the King of England, then as a widow, remarried Cnut, the Dane who also became King of England. It’s through her that the Normans formed a claim on England.