The first Viking attack up the river Seine took place in 820. By 911, the area had been raided many times and there were even small Viking settlements on the lower Seine. The text of the Treaty of Saint-Clair-sur-Epte has not survived. It is only known through the historian Dudo of Saint-Quentin, who was writing a century after the event. The exact date of the treaty is unknown, but it was likely in the autumn of 911. By the agreement, Charles III, king of the West Franks, granted to the Viking leader Rollo some lands along the lower Seine that were apparently already under Danish control. Whether Rollo himself was a Dane or a Norwegian is not known. For his part, Rollo agreed to defend the territory from other Vikings and that he and his men would convert to Christianity.
The territory ceded to Rollo comprised the pagi of the Caux, Évrecin, Roumois and
Talou. This was territory formerly known as the county of Rouen, and which would become Upper Normandy. A royal diploma of 918 confirms the donation of 911, using the verb adnuo ("I grant"). There is no evidence that Rollo owed any service or oath to the king for his lands, nor that there were any legal means for the king to take them back: they were granted outright. Likewise, Rollo does not seem to have been created a count or given comital authority, but later sagas refer to him as Rúðujarl (earl of Rouen).
In 924, King Radulf extended Rollo's county westward up to the river Vire, including the Bessin, where some Danes from England had settled not long before. In 933, King Radulf granted the Avranchin and Cotentin to Rollo's son and successor, William Longsword. These areas had been previously under Breton rule. The northern Cotentin had been settled by Norwegians coming from the region of the Irish Sea. There was initially much hostility between these Norwegian settlers and their new Danish overlords. These expansions brought the boundaries of Normandy roughly in line with those of the ecclesiastical province of Rouen.
The Norman polity had to contend with the Frankish and Breton systems of power that already existed in Normandy. In the early 10th century, Normandy was not a political or monetary unit. According to many academics, "the formation of a new aristocracy, monastic reform, episcopal revival, written bureaucracy, saints’ cults – with necessarily different timelines" were as important if not more than the ducal narrative espoused by Dudo. The formation of the Norman state also coincided with the creation of an origin myth for the Norman ducal family through Dudo, such as Rollo being compared to a "good pagan" like the Trojan hero Aeneas. Through this narrative, the Normans were assimilated closer to the Frankish core as they moved away from their pagan Scandinavian origins.
There were two distinct patterns of Norse settlement in the duchy. In the Danish area in the Roumois and the Caux, settlers intermingled with the indigenous Gallo-Romance-speaking population. Rollo shared out the large estates with his companions and gave agricultural land to his other followers. Danish settlers cleared their own land to farm it, and there was no segregation of populations.
In the northern Cotentin on the other hand, the population was purely Norwegian. Coastal features bore Norse names as did the three pagi of Haga, Sarnes and Helganes (as late as 1027). The Norwegians may even have set up a þing, an assembly of all free men, whose meeting place may be preserved in the name of
Within a few generations of the founding of Normandy in 911, however, the Scandinavian settlers had intermarried with the natives and adopted much of their culture. But in 911, Normandy was not a political nor monetary unit. Frankish culture remained dominant and according to some scholars, 10th century Normandy was characterized by a diverse Scandinavian population interacting with the "local Frankish matrix" that existed in the region. In the end, the Normans stressed assimilation with the local population. In the 11th century, the anonymous author of the Miracles of Saint Wulfram referred to the formation of a Norman identity as "shaping [of] all races into one single people".
According to some historians, the idea of "Norman" as a political identity was a deliberate creation of the court of Richard I in the 960s as a way to "to create a powerful if rather incoherent sense of group solidarity to galvanize the duchy's disparate elites around the duke".
Starting with Rollo, Normandy was ruled by an enduring and long-lived Viking dynasty. Illegitimacy was not a bar to succession and three of the first six rulers of Normandy were illegitimate sons of concubines. Rollo's successor, William Longsword managed in expanding his domain and came into conflict with Arnulf of Flanders, who had him assassinated in 942. This lead to a crisis in Normandy, with a minor succeeding as Richard I and also lead to a temporary revival of Norse paganism in Normandy. Richard I's son, Richard II was the first to be styled duke of Normandy, the ducal title becoming established between 987 and 1006.
The Norman dukes created the most powerful, consolidated duchy in Western Europe between the years 980, when the dukes helped place Hugh Capet on the French throne, and 1050. Scholarly churchmen were brought into Normandy from the Rhineland, and they built and endowed monasteries and supported monastic schools. thus helping to integrate distant territories into a wider framework. The dukes imposed heavy feudal burdens on the ecclesiastical fiefs, which supplied the armed knights that enabled the dukes to control the restive lay lords but whose bastards could not inherit. By the mid-11th century the Duke of Normandy could count on more than 300 armed and mounted knights from his ecclesiastical vassals alone. By the 1020s the dukes were able to impose vassalage on the lay nobility as well. Until Richard II, the Norman rulers did not hesitate to call Viking mercenaries for help to get rid of their enemies around Normandy, such as the king of the Francs himself. Olaf Haraldsson crossed the Channel in such circumstances to support Richard II in the conflict against the count of Chartres and was baptized in Rouen in 1014.
In 1066, Duke William defeated Harold II of England at the Battle of Hastings and was subsequently crowned King of England, through the Norman conquest of England. Anglo-Norman and French relations became complicated after the Norman Conquest. The Norman dukes retained control of their holdings in Normandy as vassals owing fealty to the King of France, but they were his equals as kings of England. From 1154 until 1214, with the creation of the Angevin Empire, the Angevin kings of England controlled half of France and all of England, dwarfing the power of the French king, yet the Angevins were still technically French vassals.
The Duchy remained part of the Anglo-Norman realm until 1204, when Philip II of France conquered the continental lands of the Duchy, which became part of the royal domain. The English sovereigns continued to claim them until the Treaty of Paris (1259) but in fact kept only the Channel Islands. Having little confidence in the loyalty of the Normans, Philip installed French administrators and built a powerful fortress, the Château de Rouen, as a symbol of royal power.
Although within the royal demesne, Normandy retained some specificity. Norman law continued to serve as the basis for court decisions. In 1315, faced with the constant encroachments of royal power on the liberties of Normandy, the barons and towns pressed the Norman Charter on the king. This document did not provide autonomy to the province but protected it against arbitrary royal acts. The judgments of the Exchequer, the main court of Normandy, were declared final. This meant that Paris could not reverse a judgment of Rouen. Another important concession was that the King of France could not raise a new tax without the consent of the Normans. However the charter, granted at a time when royal authority was faltering, was violated several times thereafter when the monarchy had regained its power.
The Duchy of Normandy survived mainly by the intermittent installation of a duke. In practice, the King of France sometimes gave that portion of his kingdom to a close member of his family, who then did homage to the king. Philippe VI made Jean, his eldest son and heir to his throne, the Duke of Normandy. In turn, Jean II appointed his heir, Charles.
In 1465, Louis XI was forced by the League of the Public Weal to cede the duchy to his eighteen-year-old brother, Charles de Valois. This concession was a problem for the king since Charles was the puppet of the king's enemies. Normandy could thus serve as a basis for rebellion against the royal power. In 1469, therefore, Louis XI convinced his brother under duress to exchange Normandy for the Duchy of Guyenne (Aquitaine). Finally, at the request of the cowed
Estates of Normandy and to signify that the duchy would not be ceded again, at a session of the Norman Exchequer on 9 November 1469 the ducal ring was placed on an anvil and smashed. Philippe de Commynes expressed what was probably a common Norman thought of the time: "It has always seemed good to the Normans and still does that their great duchy really should require a duke" (A tousjours bien semblé aux Normands et faict encores que si grand duchié comme la leur requiert bien un duc).
Dauphin Louis Charles, the second son of Louis XVI, was again given the nominal title of 'Duke of Normandy' before the death of his elder brother in 1789.