Outside, peace among Christian princes is the great purpose of his policy. He willingly declares:"Blessed be the peacemakers" and poses as the arbiter of Christianity.
When he takes over the government of the kingdom, the quarrel between the Priesthood and the Empire is in a critical phase and, faced with this problem, it will never depart from the policy it adopts immediately; deference to the Holy See, good will towards the Emperor, firm intention to safeguard against the two belligerents the rights and interests of the crown of France, but also a tenacious desire to obtain pacification through its mediation.
He must also intervene to maintain peace between the pretenders to the succession of Flanders and Hainaut, the Dampierres and the Avesnes. By a first arbitration rendered in 1246, he granted the county of Flanders to the former and Hainaut to the latter.
But, on his return from Palestine, he imposed on Jean d'Avesnes, who had only reluctantly submitted to his decisions, the restitution to Flanders of part of the affiliations which had been granted to it with Hainaut (Said of Péronne in 1256).
Other disputes still attract his attention, such as that aroused by the succession of Navarre. But, of all the arbitral awards that he is called upon to pronounce, there is not one that has made as much noise as the Mise d'Amiens of 1264, intended to decide between the King of England Henry III and his revolted barons. .
The history of relations between France and England under his reign is particularly characteristic of the line of conduct he adopted with regard to his neighbours.
Since his failure of 1242, Henry III of England remains quiet or thereabouts. It is neither peace nor war. But Louis IX wanted a definitive treaty to be reached. So when, in 1254, Henry III asked him for permission to cross the kingdom of France to go from Gascony to England, he eagerly consented, went to meet his host as far as Chartres and opened the negotiations which led to the treaty. of Paris of 1259.
By this agreement, Henry III once again became the liege man of the King of France as Duke of Aquitaine. He renounces Normandy, Maine, Anjou, Touraine and Poitou; but Louis IX gives him back all he had in fiefs or domains in the dioceses of Limoges, Cahors and Périgueux with, moreover, the expectation of Saintonge, Charente and Agenais, in case, following the death of Count Alphonse de Poitiers, who had no children, these lands would fall to the crown of France.
Treaty disapproved by public opinion very hostile to the English and amazed to see yielding to the once defeated enemy what the victorious enemy would have obtained with difficulty, but which was to appease for several decades the conflict opened by Philippe Auguste and Jean sans Terre. The money that Saint Louis had undertaken by the Treaty of Paris to pay to the King of England and that he pays after having raised aid for this purpose was, in his mind, intended for the crusade. In reality, Henry III employed him in the fight against his barons for the abolition of the Oxford Provisions, which restricted royal authority. The case is once again submitted to the King of France who makes his most famous arbitration on this occasion. By the Mise d'Amiens (1264), he came out for the King of England against the barons. But this time he is not listened to. The condemned party does not accept its sentence. Civil war breaks out in England.
Vis-à-vis the kings of Aragon who also had claims on several provinces of France, Louis IX adopted the same policy as that which he led with regard to England. In the same spirit, a compromise was signed (Treaty of Corbeil in 1258) by which Louis IX renounced all rights over Roussillon and Barcelona, subject to the abandonment by Aragon of its claims to Provence and Languedoc, to the exception of Narbonne.
But the peaceful bias of the king is put to the test on the side of Italy. We cannot attribute to her the error with the incalculable consequences of the intervention of her brother Charles of Anjou in Italian affairs (she inaugurated the deplorable expeditions of the "royals of France" to Italy which wasted energy and resources of France and will contribute to compromise its destinies in the Netherlands and on the Rhine).
But he lets her commit and he will quickly regret seeing the zeal of the popes and the elite of his chivalry to expend in a war in which flows, on both sides, Christian blood.