The Normans used the deposition of the previous emperor Michael by Nicephorus Botaneiates as the casus belli to invade the Balkans. This gave Robert a motive to invade the empire claiming his daughter had been mistreated.

The Battle of Dyrrhachium was fought between the Byzantine Empire, led by the Emperor Alexios I Komnenos, and the Normans of southern Italy under Robert Guiscard, Duke of Apulia and Calabria. The battle ended in a Norman victory and was a heavy defeat for Alexios. Historian Jonathan Harris states that the defeat was "every bit as severe as that at Manzikert." He lost about 5,000 of his men, including most of the Varangians. Norman losses are unknown, but John Haldon claims they are substantial as both wings broke and fled.