The Arendish Civil War is a long-running conflict that tore Arendia into multiple factions for millennia.
The exact origins of the hostilities that have always existed between the duchies are mired in obscurity; no sooner was Arendia founded than its mix of feudalism and knighthood began turning it into a hotbed of feuds and grudges. However, the scholars at the University of Tol Honeth have speculated that the original casus belli for the civil war came in the second millennium, when Asturia expanded to the eastern coast with Vo Aldurin and Vo Kriederin while simultaneously building fortifications at the southern bank of the Astur River (against the Wacites) and the southern edge of the Arendish Forest (against the Mimbrates.) This effectively bisected the country, and naturally, Wacune and Mimbre declared war. However, a full-on war was not feasible; Asturia's fortifications were too strong, and neither the Wacites nor Mimbrates could put up their full forces; the Wacites were engaged in a war with Cherek at the time, while the Mimbrates were in the middle of a centuries-long war with Tolnedra.
However, in 2618, the Duke of Asturia declares himself the King of Arendia, and demanded that his counterparts in Wacune and Mimbre pay homage. While some more charitable scholars have suggested that this was all a ploy to get the other duchies to abandon their foreign wars, the actual result was that the Dukes of Wacune and Mimbre each declared themselves to be King. In the ensuing confusion, Cherek managed to take a bite out of Wacune, while Tolnedra forced the Mimbrates to withdraw to their side of the River Arend. Humiliated abroad, the two duchies looked for someone to blame, and naturally settled on Asturia. And thus the Arendish Civil War began in earnest.