[8] In 1199, while taking part in a tournament at Ecry-sur-Aisne, he took the cross in the company of Count Thibaud de Champagne and went on the Fourth Crusade.

He and his associates, including Abbot Guy of Vaux-de-Cernay, left the crusade when the decision was taken to divert once more to Constantinople to place Alexius IV Angelus on the throne.

Simon was rewarded with the territory conquered from Raymond VI of Toulouse, which in theory made him the most important landowner in Occitania.

Simon's part in the crusade had the full backing of his feudal superior, the King of France, Philip Augustus.

However, historian Alistair Horne, in his book Seven Ages of Paris, states that Philip "turned a blind eye to Simon de Montfort's crusade ... of which he disapproved, but readily accepted the spoils to his exchequer".

Following the latter's success in winning Normandy from John Lackland of England, he was approached by Innocent III to lead the crusade but turned this down.

He also sought to counter any adventure by King John of England, who had marriage and fealty ties also with the Toulouse comtal house.

[citation needed] Simon is described as a man of unflinching religious orthodoxy, deeply committed to the Dominican order and the uprooting of heresy.

Simon had other key confederates in this enterprise, which many historians view as a conquest of southern lands by greedy men from the north.

One was Guy Vaux de Cernay, head of a Cistercian abbey not more than twenty miles from Simon's patrimony of Montfort Aumary, who accompanied the crusade in the Languedoc and became bishop of Carcassonne.

[citation needed] Simon was an energetic campaigner, rapidly moving his forces to strike at those who had broken their faith with him – and there were many, as some local lords switched sides whenever the moment seemed propitious.

Simon hastened to besiege the city, meanwhile sending his wife, Alix de Montmorency, with bishop Foulques of Toulouse and others, to the French court to plead for support.

His head was smashed by a stone from a mangonel, operated, according to one source, by the donas e tozas e mulhers ("ladies and girls and women") of Toulouse.