During the latter years of the elderly Charlemagne's rule, the Vikings made advances along the northern and western perimeters of his kingdom. After Charlemagne's death in 814 his heirs were incapable of maintaining any kind of political unity and the once great Empire began to crumble. Viking advances were allowed to escalate, their dreaded longboats were sailing up the Loire and Seine Rivers and other inland waterways, wreaking havoc and spreading terror. In 843 the Viking invaders murdered the Bishop of Nantes and a few years after that, they burned the Church of Saint-Martin at Tours. Emboldened by their successes, in 845 the Vikings ransacked Paris.

During the reign of Charles the Simple (898-922) whose territory comprised much of the France of today, he was forced to concede to the Vikings a large area on either side of the Seine River, downstream from Paris, that was to become Normandy.

The Carolingians were subsequently to share the fate of their predecessors: after an intermittent power struggle between the two families, the accession (987) of Hugh Capet, duke of France and count of Paris, established on the throne the Capetian dynasty which with its Valois and Bourbon offshoots was to rule France for more than 800 years.

The Carolingian era had seen the gradual emergence of institutions which were to condition France's development for centuries to come: the acknowledgement by the crown of the administrative authority of the realm's nobles within their territories in return for their (sometimes tenuous) loyalty and military support, a phenomenon readily visible in the rise of the Capetians and foreshadowed to some extent by the Carolingians' own rise to power.

The new order left the new dynasty in immediate control of little beyond the middle Seine and adjacent territories, while powerful territorial lords such as the 10th and 11th-century counts of Blois accumulated large domains of their own through marriage and through private arrangements with lesser nobles for protection and support.

The area around the lower Seine, ceded to Scandinavian invaders as the duchy of Normandy in 911, became a source of particular concern when duke William took possession of the kingdom of England in the Norman Conquest of 1066, making himself and his heirs the king's equal outside France (where he was still nominally subject to the crown).

Worse was to follow. A protracted succession dispute among William's descendents ended in 1154 with the coronation of Henry II. Henry had inherited the duchy of Normandy and the county of Anjou from his father, Geoffrey of Anjou, and in 1152, he had married France's newly-divorced ex-queen, Eleanor of Aquitaine, who ruled much of southwest France. After deafeating a revolt led by Eleanor and three of their four adult sons, Henry had Eleanor imprisoned, made the duke of Brittany his vassal, and in effect ruled the western half of France as a greater power than the French throne. However, disputes between Henry's descendents over the division of his French territories, coupled with Richard I's lengthy absence during, and imprisonment while returning from, the Third Crusade, allowed Philip II of France to recover influence over most of this territory. After the French victory at the Battle of Bouvines in 1214, the English monarchs maintained power only in southwestern duchy of Guyenne.

The 13th century was to bring the crown important gains also in the south, where a papal-royal crusade against the region's Albigensian or Cathar heretics (1209) led to the incorporation into the royal domain of Lower (1229) and Upper (1271) Languedoc. Philippe IV's seizure of Flanders (1300) was less successful, ending two years later in the rout of her knights by the forces of the Flemish cities at the Battle of the Golden Spurs near Courtrai (Kortrijk).