Louis IX--later known as St. Louis--came to power on his father's death in 1226. A child, his regent was Blanche of Castile. Seeing a regency under a woman's administration as an ideal time for revolt, barons in Brittany, and southern France that had been held by the Plantagenets revolted, supported by Henry III of England. Blanche was able to put down these revolts due to her intelligence and the large royal army built up by Philip Augustus and Louis VIII. The rest of St. Louis'; reign was one of comparative peace. The 1229 Treaty of Paris ended the Albigensian Crusades. The Count of Toulouse ceded the northern portions of his lands to the Crown directly, with the rest of it becoming royal lands upon his daughter's death in 1271. In 1249, Louis took the Cross and went on Crusade to recapture Jerusalem, which had fallen to Khwarazmshah warriors fleeing the Mongol advance westward. It was the major failure of his reign. Planning to advance on Jerusalem through Egypt, Louis' great army did indeed capture Damietta, but then got bogged down in the Nile Delta. They were defeated by the Egyptian forces, commanded by Mamluks and under the nominal control of the last Ayyubid rulers. Louis was captured, and obtained his and his troops freedom only upon payment of a huge ransom. He then spent until 1255 in Palestine, but was not able to recover the Holy City. Back home, in the 1259 Peace of Paris, England's Henry III was made to definitively and finally renounce claims to Normandy, Maine, Anjou and Poitou, keeping Gascony only as a vassal to Louis IX. This ended any English hope of reviving the Angevin legacy. Louis' death came in 1270. Going on Crusade again with his brother/rival Charles of Anjou, he became ill shortly after landing in Tunis, whose Amir had said he would convert if a large Christian army came. The Amir did not keep his word, and Louis died shortly thereafter.
St. Louis is also significant for internal policies. When he came into his majority, the baillis set up by his predecessors were being increasingly accused of extortion and unjust activities regarding the masses. He thus began to send out enqueteurs--inquestors--who were royal officials empowered to travel throughout the realm investigating conduct of local officials and looking into complaints. Further, Louis encouraged the right of appeal to the crown. This meant that the juridical responsibilities of the monarch increased. In this case an itinerant court would no longer do. A permanent royal court with regular sessions began to meet in Paris, staffed by a permanent cadre of royal officials, upon whom Louis would check depending upon the importance of the case. This court became known as the parlement.
As well, Louis jealously guarded the rights of the crown vis-à-vis the barons, and expressed this mostly in opposition to feudal warfare. He put increasing conditions on war between feudal wars, including the necessity of acquiescing to truces, allowing relatives of the parties to opt out of hostilities, etc., before finally prohibiting private war totally and disallowing nobles from leading armed bands around their domains. Beyond this, his predecessors' acquisition of France's richest areas meant that he could afford a large army and police force, did not need to tax nobles, and could boss them around given the prestige of the throne. In three centuries, then, France had matured into a powerful feudal monarchy.
In every case that a child-king came to a medieval throne and ruled through a regent, political instability resulted. This was particularly the case in 11- 12th century Germany, and even more so when a woman was the chief or sole regent. Why, then, was Louis IX able to escape the same fate? 1) The prestige of the French monarchy by 1226, after Bouvines and the acquisition of important lands was such that the institution itself possessed moral authority, as well as the funds, to fight off challenges. For more and more lesser feudal lords, it simply made sense to support the crown, since by doing so they were supporting themselves. 2) Henry III could not fund the revolts sufficiently. The royal domain in Britain had been shrinking ever since William II, and the King was now near bankruptcy, especially after losing Angevin lands. 3) In the same vein, while medieval armies including those of France were increasing in size from the late twelfth century, England's was shrinking, with the monarchy able to afford fewer knights, and not having the funds for mercenaries. Also, as several English barons did not support Henry's campaign against St. Louis, they were not willing to provide soldiers. 4) Blanche was an able leader with good military advisers, whereas Henry was poor at soldiering.
Significant about Louis IX are the images of him. His cultivated persona was one of total Christian virtue and fairness to all his subjects. He succeeded not by overturning feudal relations, but by insisting upon their equitable continuation. Allowing Henry III to keep Gascony as a fief was cited by the king's supporters as an act of feudal equity and balance. As far as justice is concerned, stories are related of Louis holding informal court under a great oak tree in the Vincennes forest, where anyone could come to have a wrong redressed. He was also deeply pious, as reflected in his monastic patronage, crusades to the east as well as against Albigensians in France, and--regrettably but totally in character for a medieval Christian--in his oppression of French Jewry. St. Louis, then would become the paragon of kingship for later medieval Frenchmen and leaders.
There was a certain price for this, however. Louis did very little to unite the kingdom beyond being a patchwork of feudal territories whose leaders recognized his overall authority. In some regions lords had their own governments, court systems, armies, and even tolls. Though these lords were the king's vassals, it was not always the case that their vassals saw themselves as subordinate to Louis as well. Some of these second-echelon feudal lords would fight against the king with a clear conscience if they felt their prerogatives had been violated. Thus, the king ruled France through local feudal rulers who were quite powerful in their own right, the basic glue for the system being the esteem in which they held the overall feudal lord--the king. Individual nobles never developed the ability to cooperate with each other. More importantly, no new political technology emerged, such that concepts valid in the Carolingian period--individual lord-vassal bonds; the secular-spiritual power of kings; and alliance with the church--were still powerful in 1270s France.