Louis IX became king of France in 1226, with his mother Blanche of Castile governing during his minority and defending the monarchy against aristocratic challenges. As an adult he strengthened royal authority through military success, administration, and a reputation for justice. Victory over opponents supported by Henry III of England in 1242 consolidated his position in western France. The Treaty of Paris in 1259 later settled important disputes with the English crown while preserving English holdings under French overlordship.

Louis sought to make royal government answerable to Christian moral standards. He used inquiries into official misconduct, expanded royal judicial authority, and restricted private warfare. Litigants could seek his justice beyond local lordships. His piety supported hospitals, religious houses, and patronage of mendicant orders. The Sainte-Chapelle in Paris housed relics of Christ's Passion and joined royal devotion to a powerful architectural statement of sacred kingship.

That religious program included persecution as well as charity. Louis supported the condemnation and burning of Jewish religious books and imposed discriminatory restrictions on Jews. His ideal of a Christian kingdom did not entail religious equality. Crusading became another central commitment. In 1248 he led an expedition against Egypt, capturing Damietta before defeat and capture in 1250. Ransomed, he remained in the eastern Mediterranean for several years, strengthening defenses and negotiating on behalf of the remaining crusader states.

Returning to France in 1254, he resumed government while preparing for another expedition. In 1270 he led a crusade to Tunis, where disease devastated the camp and he died. His son Philip III succeeded him. The church canonized Louis in 1297, and subsequent Capetian rulers drew authority from association with a royal saint. His conception of Christian monarchy shaped both administrative consolidation and coercive religious policies.