/ Avignon's surrender to Louis VIII (1226) — BnFThe crusade against the Cathars, launched in 1209, transformed Languedoc and gradually strengthened the French crown's influence in the south. It began before King Louis VIII's personal intervention: he led his own campaign in 1226 against territories associated with Raymond VII, count of Toulouse.
Louis VIII marched down the Rhône Valley with his army. Avignon, an ally of the count of Toulouse, refused to open its gates. The king besieged the city, which surrendered in September 1226 after around three months of resistance.
The campaign strengthened the French monarchy's position in the south. Dynastic ties also grew closer when Margaret of Provence married Louis IX, the future Saint Louis.
Sources and further reading: City of Avignon: the walls and the siege of 1226.