The siege of Avignon was the principal military action of the Albigensian Crusade of 1226. King Louis VIII of France besieged the town of Avignon, which lay within the Holy Roman Empire, from 10 June until 9 September, when it surrendered on terms.
Background
Louis VIII assembled the largest army of the Albigensian Crusades at Bourges in May 1226. He advanced to Lyon and then down the Rhône Valley on the left bank (the imperial side), his ultimate goal being the submission of Count Raymond VII of Toulouse, who as Marquis of Provence also held lordship over Avignon. The latter was an autonomous city, governed by a podestà and consuls on the Italian model. It was wealthy, possessing a full double wall with two large gate towers (named Quiquenparle and Quiquengrogne). Its defences were manned by mercenary soldiers. It had, however, been under an interdict for twelve years for having refused an order of the pope.
Louis's plan was to cross back into France at Avignon. Representatives of Avignon appeared at Bourges in order to reach an understanding with Louis and prevent any violence to their city. They agreed not to impede the travel of Louis's baggage down the Rhône, and to allow him to cross the stone bridge of Saint-Bénézet (completed in 1188) with a small retinue. Louis could thus enter the city, but the majority of the army would have to go around the walls and avoid the bridge. Count Raymond Berengar IV of Provence also came to an understanding with Louis to prevent any damage to his lands.
Dispute between the king and the city
When Louis arrived at Sorgues six miles from the city, an Avignonese magistrate greeted him and ceded him the city of Beaucaire across the river from Avignon, which belonged to Raymond VII, but which the count had mortgaged to Avignon to finance his resistance. In exchange, Louis paid Raymond's debt. The Avignonese also asked for absolution from their excommunication and requested that the papal legate, Cardinal Romanus of Sant'Angelo, enter the city with the other bishops and Louis accompanied by only 100 knights to lift the ban and receive the oaths of the Avignonese. These terms were accepted and the magistrate left behind hostages as a guarantee of good faith. According to Philippe Mousket, the 50 hostages given by the city turned out to be poor men passed off as the sons of the bourgeoisie. At Sorgues, the crusading army was spread out over four miles of riverbank.
Louis reached Avignon on 7 June. At this point there seems to have been some confusion. To prevent the army from trying to use the stone bridge, the citizens built a wooden (possibly pontoon) bridge outside the city, which Louis's vanguard crossed without incident. The king, for reasons of prestige, refused to use it and demanded the fulfillment of the agreement. When the contingent under Lord Walter II of Avesnes reached the makeshift bridge, after having marched under the walls of the city with banners flying, it was attacked by a party from the city. Several crusaders were killed and several captured. The rest crossed the bridge to safety.
The reasons for the Avignonese attack are unknown, but all the French sources and the papal legate's report agree that the Avignonese were the instigators. According to one rumour popular at the time, the Avignonese intended to kill Louis and the papal legate while they were passing through the city. This, however, is inconsistent with the voluntary handing over of Beaucaire and may be dismissed as a mere rumour. The sole contemporary source to blame Louis for the break with Avignon is Roger of Wendover, who accuses the king of planning all along to attack the city.
When Louis demanded the city abide by its agreement, the consuls accused him of having violated it himself. It is possible that Walter of Avesnes's movements were mistaken for the preparation of an assault. Avignon blocked the Rhône and withheld the provisions that Louis had purchased without refunding him his money. The legate sent some Dominicans into Avignon to demand compliance with the agreement, but to no avail.