The Siege of Ragusa (modern Dubrovnik in Croatia) by the Aghlabids of Ifriqiya lasted for fifteen months, beginning in 866 until the lifting of the siege at the approach of a Byzantine fleet in 868. The failure of the siege and the re-appearance of the Byzantines in the region of Dalmatia signalled the beginning of new aggressive western policy by the new Byzantine emperor, Basil I. Its immediate effects were the re-establishment of Byzantine authority there in the form of the Theme of Dalmatia, and the beginning of the Christianization of the Slavs of the western Balkans, but within a few years it led to renewed Byzantine involvement and presence in southern Italy as well.