In 1208, Raymond VI, Count of Toulouse, was one of the most powerful monarchs in Europe. He had vast holdings covering most of southern France. His domain was not French—the inhabitants spoke a different language, the Langue d'Oc, now called Old Occitan. Raymond wasn't bothered much by allegiance to the Catholic Church, and allowed people of all religions act as their conscience dictated. Jews, Catholics, and heretics coexisted.
Pope Innocent III
This last group, the heretics, caught the notice of Pope Innocent III. That and the fact that the papal legate, Brother Peter of Castelnau, was assassinated by one of Raymond VI's officers, pushed the pope to declare the first ever crusade against a country of Christians. By then, the pope had already written to the King of France urging him to overthrow Raymond, primarily because his country did not recognize the dominion of the Catholic Church centered in Rome.
Crusaders were volunteers. The church couldn't command an army, so they promised salvation instead, and the prospect of glory and riches didn't hurt either. The crusade against the heretics in Raymond VI's domain attracted French fighters from the north, as well as French sympathizers who lived in the Midi.
The French King
Interestingly enough, King Philip of France didn't really want this crusade. He was busy trying to fend off English aggression, and not eager to put himself or his son in harm's way. Nonetheless, plenty of French barons joined in, as did church leaders. Bishops, at this time, could be warriors.
Life in the Midi
As hinted above, this part of Europe enjoyed a very different kind of social order than the northern regions. In addition to nobles, a thriving bourgeoisie inhabited cities with rich culture and lively trade. With the exception of dangerous main roads, peasants and citizens could live in relative safety and prosperity. The remarkable culture of the Trobadors and Trobairitz—nobles who exchanged courtly love poems and songs—played out against a relatively stable, peaceful backdrop. Although nominally a Christian realm, the problem was that because the church had so little power, the heretics became almost as numerous as the Catholics, so eradicating them was not only impractical but undesirable.