In Guernsey, the British monarch is still officially Duke of Normandy
Normandy began as a deal: in 911 a Frankish king handed land on the lower Seine to a Viking chief called Rollo, on condition that he guard it against other raiders and accept baptism. France took the mainland back in 1204, but the Channel Islands never left, and their ruler still carries the ducal title.
Scandinavian raiders first rowed up the Seine in 820. They plundered monasteries such as Jumieges, forced Rouen to be abandoned temporarily in 841, and reached Paris in 845 under a leader recorded as Ragnar. Over time some began wintering in fortified camps on the lower river. In 911, after Frankish forces relieved Rollo's siege of Chartres, King Charles the Simple chose to negotiate, and the Treaty of Saint-Clair-sur-Epte made the settlers official. The region's old name, Neustria, gave way to one taken from its newcomers, the Nortmanni, men of the North.
Rollo's heirs expanded west to the Vire and the Cotentin by 933. His son William Longsword was assassinated in 942 at the instigation of Arnulf of Flanders, briefly reviving paganism, and the formal ducal title only settled in between 987 and 1006 under Richard II. Settlement varied: Danes in the Caux mixed freely with the Gallo-Roman locals, while Norwegians in the northern Cotentin resisted Christianity and kept speaking Old Norse for generations. Some historians argue that a unifying Norman identity was deliberately crafted at Richard I's court in the 960s, backed by an origin myth likening Rollo to Aeneas.
Between 980 and 1050 the dukes built the strongest, most tightly run duchy in western Europe, keeping vassals in check and even calling in Viking allies; the future Norwegian king Olaf Haraldsson was baptised in Rouen in 1014. Richard II's sister Emma married England's King Ethelred II, a link that foreshadowed 1066, when Duke William defeated Harold II at Hastings. Thereafter the dukes were kings in England while remaining vassals of the French crown in Normandy.
That awkward arrangement ended when Philip II of France declared the duchy forfeit in 1202 and seized it two years later, raising a fortress at Rouen to assert his rule. England abandoned its mainland claim in the Treaty of Paris of 1259, keeping only the islands, where no formal end to the duke's rule has ever been declared.
Source: Duchy of Normandy
