 To the north , the Bulgar frontier remained quiet , except for a Bulgar raid that was defeated , leading to the renewal of the 30 @-@ year peace treaty of 815 , which was later reconfirmed by the new Bulgar khan Boris ( r . 852 – 889 ) . Byzantium thus enjoyed a period of peace except in the West , where the Byzantine government proved unable to halt the ongoing Muslim conquest of Sicily . Modica fell in 845 , but although Constantinople used the relative quiet in the East to send reinforcements to the island , these were heavily defeated at Butera , where the Byzantines lost about 10 @,@ 000 men . In the wake of this disaster , Leontini in 846 and then Ragusa in 848 fell to the Muslims , while an attempt by the Byzantine fleet to land troops near Palermo in winter 847 / 848 failed . Over the next few years , the Muslims raided the Byzantine territories on the eastern half of the island unopposed , capturing several minor fortresses and securing ransom and prisoners from others . 
