Following the Norman conquest of Byzantine Italy and Saracen Sicily, the Byzantine emperor, Michael VII Doukas (r. 1071–1078), betrothed his son to Robert Guiscard's daughter. When Michael was deposed, Robert took this as an excuse to invade the Byzantine Empire in 1081. His army laid siege to Dyrrhachium, but his fleet was defeated by the Venetians. On October 18, the Normans engaged a Byzantine army under Alexius I Comnenus outside Dyrrhachium. The battle began with the Byzantine right wing routing the Norman left wing, which broke and fled. Varangian mercenaries joined in the pursuit of the fleeing Normans, but became separated from the main force and were massacred. Norman knights in the centre attacked the Byzantine centre and routed it, causing the bulk of the Byzantine army to rout.