 With the decline of the Caliphate and its obvious inability to defend its border provinces , a new local dynasty , the Hamdanids , emerged as the principal antagonists of Byzantium in northern Mesopotamia and Syria . They were led by al @-@ Hasan , called Nasir al @-@ Dawla ( " Defender of the State " ) , and by his younger brother Ali , best known by his epithet , Sayf al @-@ Dawla ( " Sword of the State " ) . In ca . 935 , the Arab tribe of Banu Habib , defeated by the rising Hamdanids , defected in its entirety to the Byzantines , converted to Christianity , and placed its 12 @,@ 000 horsemen at the disposal of the Empire . They were settled along the western bank of the Euphrates and assigned to guard five new themes created there : Melitene , <unk> , <unk> ( Arsamosata ) , <unk> , and <unk> . 
