Launched on the grounds of forcing Occitan nobles to stop sheltering Cathar heretics, the Albigensian Crusade in effect quickly became an attempt by the chief crusader Simon de Montfort (father of the Simon de Montfort famous from English history) to take over the lands of the count of Toulouse and other southern nobles. For a long time he, despite a desperate lack of money forcing him to rely on volunteer crusaders serving 40 days for an indulgence, was remarkably successful, but from 1216 a southern rebellion undid much of his work and he was killed in 1218 at the 2nd siege of Toulouse. After his death the crusade was taken over by the French monarchy who used it to turn Occitania into a part of the French kingdom in fact as well as in name - in the twelfth century it had been for all practical purposes independent.

Marvin offers a chronological narrative mostly following the chief crusader - or "athlete of Christ" as he often refers to him - campaigning back and forth between Provence and Gascony. He ranks Montfort as the best commander of either side and judges him an examplar of the values of his time and social class; while not neglecting to point out that those are not modern values. He discusses the infamous atrocities commited by the crusaders, such as the sack of Béziers (where the papal legate is supposed to have said "kill them - God will know his own" in response to what should be done with prisoners who might be good Christians or dissembling heretics), but also reminds us that the other side also commited atrocities and that what we'd call crimes against humanity were routine in 13th century warfare. While flashes of personal judgment occur, the overall tone is of academic detachment.

The unexpected villain of the book is pope Innocent III, who launched the crusade but then failed to either support it wholeheartedly or decisively end it. Far from the strong personality one has read of elsewhere, he comes across as an indecisive leader prone to agree with whomever he spoke last.

Gives an insight into 13th century warfare that should not be missed by the serious student.

While the Albigensian Crusade has gotten a lot of attention through the years, the emphasis has always been on the Heresy itself, the Cathars, the Inquisition in place in later years etc.
This is (I believe) the first published book concerning itself primarily with military aspects of the crusade.
In naming the book 'The Occitan War', the author considers that the crusade took on a political bent very early on, essentially becoming a war of conquest, only sometime under the aegis and/or with the blessing of the Pope. With the entry of Aragon into the war, matters became more like a typical frontier war between two neighbouring kingdoms.
Marvin concentrates on the dominant features of medieval warfare; raids and sieges, and while he is happy to spend time on the few battles which occured he places them in the proper context, usually in relief of castles under siege themselves.
Marvin also considers the aspect of brutality during the war. Placing incidents in the wider context of medieval warfare, nothing can be found that is outside the bounds of the laws of war, or that wasn't done elsewhere. But the sheer frequency of these incidents would suggest that the war was unusually bitter and brutal.
The author doesn't neglect the wider political aspects of the war either. The Pope's failure to give a firm guiding hand to the crusade leads to two things: Simon de Montfort becomes a magnate in the region, using the crusade (consciously or not) as an excuse to further his own holdings; and parties associated with the Cathars continue their struggle when all seemed lost because the Pope failed to unequivocally deny them hope.
With the history finishing with Simon de Montfort's death, Marvin gives us a brief overview of the rest of the conflict, dominated by France and the Inquisition. I foresee the need for more studies, both on the later part of the war and on specific aspects/events of the war. ( )