The war is being fought by two factions: the Inquisition along with it's autonomous region, The Triplici Consulate of the Waste along with the Populares Faction composed of 4 Pillar Houses that supported Tiberius Caesar's rule. With the opposition being the Optimates Faction of the Ars Goetia Government composed of 20 Pillar Houses aimed to oust Tiberius Caesar and establish a Reformed Purists government supported by Sirzechs Lucifer, one of the 4 Overlords.
The trigger for the war was the execution of Rias Gremory, heir of the Gremory Clan and sister of Sirzechs Lucifer. This set of an ongoing war within the former capital of Lucifaad between the Inquisition and the Optimates.
Military Dictatorship Edit
Due to the Defiance 200 years ago, Tiberius Caesar placed the Devil Underworld in a Military Dictatorship with the Four Maous and Pillar Clans only ruling only in name. This led to the Inquisition as the only standing army within the Devil Underworld.
Political Differences Edit
The Defiance had shifted the political power from the Pure-Blooded Devils to the Common Devils. Although the Ars Goetia Parliament still is controlled by the Pillar Housed. Laws and Decisions are all passed by Tiberius with his position as the High Judge of the Underworld. With their different views on how to run the newly formed government, the Purists and Commons always been in a political stalemate
Divide in the Underworld Edit
One of the more obvious reason is the glaring class divide between the Devil Society, while many Common Devils support the Inquisition some of them still held loyalty to their houses.
Growing Power of the Triplici Waste Edit
The Devil Civil War had radically altered the political Underworld map, with Tiberius and the Seekers were granted the of the Waste —and the years after the founding of the Triplici Waste. The Pillar Clans were alarmed by it's growing power were too late to realize it's political threat, after the Defiance the Triplici Waste gained more and more autonomy from the Devil Underworld.
Many Purists feared that the Triplici Waste would replace the Ars Goetia Government within the Three Factions.
Resurface of the Old Maous Edit
10 years before the story ,Two Devils employed by Katarea Leviathan kidnapped the Heiress of the Sitri Clan at the same time Old Maou rebels captured five Devil cities including Kos, which culminated in the Reconquest campaign to take back the cities. Tiberius Caesar, went on to rescue the Sitri Heiress resulting in an unbreakable alliance between the Inquisition and the Sitri Clan.
Siege Of Kos Edit
The Siege of Kos was a brief conflict between the Old Maou Faction and the Inquisition. The conflict began with the reconquest of the historical Devil city by the forces of the Triplici Waste, which was launched from through the territories of the Amon Clan. The siege resulted in the destruction of the 72 Spires symbolizing the Pillar Houses and its annexation by the Inquisition army instead of handing it back to the Amon Clan; in addition, it exposed the weakness of the Ars Goetia Government as a force to preserve peace and dealt a blow to the Old Maou Faction.
Triplici Consulate allies with the Shinto Faction Edit
A Year after stopping the Nekomata Genocide, Tiberius Caesar signed a Military Alliance between Yasaka, leader of the Kyoto Youkais and representative of the Shinto Pantheon. This move surprised the denizens of the Supernatural World as the Triplici Waste strictly followed a Isolationalist policy, usually not concerning themselves to the affairs outside the Devil Underworld.
However the Optimates saw this as a threat, fearing a complete take over by Tiberius Caesar with the backing of a foreign pantheon, this added to the growing tensions between the Purists and the Inquisition.