Why did Capetian France not reach roughly its modern territorial size until the 13th century?

Early Capetian kings initially held power limited to their immediate domain around Paris, Orléans, and surrounding areas, while the rest of the kingdom was controlled by powerful regional lords who were often reluctant to acknowledge royal authority. For generations, the monarchy functioned more as a great principality, struggling to subordinate castles and vassals to royal purposes.123

The expansion toward modern territorial size occurred primarily in the 13th century due to several key factors:

• Military and Legal Consolidation: Philip II (Philip Augustus) significantly expanded the royal domain by wresting control of Normandy, Anjou, Maine, and Touraine from the English Plantagenets. He skillfully used his position as suzerain to confiscate fiefs when vassals failed to meet their obligations.43
• Administrative Reform: Philip Augustus and his successors, including Louis IX, developed more efficient administrative structures, such as bailiffs and seneschals, to govern and collect revenue in newly acquired territories.45
• Crusades and Inheritance: The Albigensian Crusade allowed the crown to extend its authority into southern France, eventually leading to the annexation of the county of Toulouse. Additionally, strategic marriages and the reversion of appanages—such as the inheritance of Alphonse of Poitiers—further consolidated the realm under the royal house.65