The war began in 1E 369 following the death of King Borgas and the failure of the Moot to appoint the capable 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 traveled 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 during the Skyrim Conquests.