The Battle of Ostrovo occurred in 1041 near Ostrovo, an area close to the lake of the same name in modern northern Greece. In 1040 Peter Delyan led an uprising against the Byzantines and was proclaimed Emperor of Bulgaria. He quickly occupied the western Balkan lands from Belgrade to Larissa but in the next year he was betrayed by his cousin Alusian, who deserted from the army and blinded him. Though blind, Peter Delyan remained in command and met the Byzantines near Ostrovo. The battle itself is unclear but the Bulgarians were defeated mainly with the help of the Varangian Guard, led by Harald Hardrada. The fate of the Bulgarian leader is also unknown; he may have perished in the battle. As a result, the uprising was crushed and Bulgaria remained under Byzantine rule until 1185.