Why did Louis IX make peace with Henry III of England?

Louis IX sought to establish a lasting peace with his brother-in-law, Henry III of England, primarily because he was a just and equitable ruler who desired to create goodwill between their respective families.1

Beyond these personal motivations, Louis was driven by a strategic vision of sovereignty. Although he held the military advantage and could have stripped Henry III of all his Continental holdings, he chose to leave him Aquitaine and neighboring territories in exchange for Henry acknowledging himself as Louis’s vassal. In the 13th century, Louis believed that a sovereign’s power was measured more by the importance and number of his vassals than by the sheer extent of his territorial possessions.1

This diplomatic effort culminated in the Treaty of Paris in 1259, which helped consolidate Louis's reign by settling long-standing territorial disputes and reinforcing the feudal hierarchy under the French crown.12