It was the Feast Day of St Mary Magdalene, 22 July 1209, and an all-out massacre had not been planned.

A French army from the north, under the leadership of the Papal legate Arnold Amaury, was camped outside the town of Béziers in the Languedoc. Recently arrived from a month-long march down the valley of the River Rhône, the army’s mission was to demand that the town elders hand over the 222 Cathars – about 10 per cent of the town’s population 1 – that they were known to be harbouring.The elders refused.That they did so says as much for the power of the Cathar faith as it does for the complicated political situation in the south in which the Cathars had been able to flourish.

The Cathars had come to prominence in the Languedoc some fifty years previously and were, by the beginning of the thirteenth century, virtually the dominant religion in the Languedoc. Unlike the majority of the Catholic clergy of the time, the Cathars were conspicuously virtuous, living lives of apostolic poverty and simplicity. This in itself would have been enough to get the sect branded as heretics, as happened to the Lyons-based group, the Waldensians.2 But what set the Cathars apart from the Waldensians was their belief in not one god, but two. According to Cathar theology, there were two eternal principles, good and evil, with the world being under the sway of the latter.They were also implacably hostile to the Church of Rome, which they denounced vehemently as the Church of Satan.

The Cathars were not the only ones to oppose Rome: most of the south of what we would today call France was fiercely independent, and regarded both the northern army and the Papal agents as foreign invaders. It was therefore unthinkable that the Cathars, fellow southerners, could be handed over to opposition. The enemy was not heresy, but anyone who challenged the authority and autonomy of the local nobility, the powerful counts and viscounts of Toulouse, Foix and Carcasonne.

The combination of heresy and politics was a combustible one, however, and Pope Innocent III (1198–1216) saw sufficient grounds to call for a Crusade. The west had been launching Crusades with varying degrees of success ever since 1095, but they had all been directed against the Muslims. Under Innocent’s pontificate, that began to change. The Fourth Crusade, launched in 1202, did not bode well for the heretics and nobles of the Languedoc: although aimed at the Holy Land, the Crusaders veered wildly off target in the spring of 1204 and sacked the fellow Christian city of Constantinople.The campaign called against the Cathars was different: it would be the first Crusade to be conducted within the west, against people who were fellow countrymen and women.
full text here
https://exploringrealhistory.blogspot.com/2020/11/part-1-cathars-most-successful-heresy.html