The war began in 1E 369 following the death of King Borgas and the moot's failure to appoint Jarl Hanse as his successor . It ended in 1E 420 when Olaf One-Eye was elected the High King of Skyrim after he captured the dragon, Numinex.
Borgas was killed during one of the Wild Hunts of Valenwood . The reason for the Bosmer's attack was that Borgas, a supporter of the Alessian Doctrines of Marukh, had travelled to Cyrodiil to urge the Alessians to launch a joint war on Valenwood.  In the aftermath of this battle, by the time Wulfharth eventually succeeded him , Skyrim had lost the majority of the territory it had earned in the Skyrim Conquests.