The reign of Alexios is well-documented due to the survival of the "Alexiad", written by his daughter Anna Komnene, which details all of the events of his reign, albeit with a bias towards Alexios. Upon ascension, Alexios inherited a much-weakened empire that was almost immediately beset by a serious invasion from the Normans of Southern Italy. The Normans used the deposition of the previous emperor Michael as the "casus belli" to invade the Balkans. Alexios did not have a strong enough army to successfully resist the invasion at first and suffered a grave defeat at the Battle of Dyrrachium (1081), which allowed Robert Guiscard and his son Bohemond to occupy much of the Balkans.