The Thousand-Year Battles were the first conflicts in the War in Heaven after the initial skirmish on Dronid. The term was somewhat of a misnomer, as only on one of the planets did it actually last for a millennium.
The battles were fought on three worlds: Utterlost, then Kaiwar and Mohandassa. On Utterlost the fighting never truly ended. No records survived about what happened to Mohandassa, and Kaiwar was abandoned when the alterations to history stripped it of any tactical value. (PROSE: The Book of the War)