z antine ▁Empire , ▁often ▁with ▁Arab ▁support . ▁With ▁the ▁support ▁of ▁C ali ph ▁Al - Ma ' mun , ▁Arab s ▁under ▁the ▁leadership ▁of ▁Thomas ▁the ▁S lav ▁inv aded , ▁so ▁that ▁within ▁a ▁matter ▁of ▁months , ▁only ▁two ▁them ata ▁in ▁Asia ▁Minor ▁remained ▁loyal ▁to ▁Emperor ▁Michael ▁II . ▁When ▁the ▁Arab s ▁captured ▁Th ess al onica , ▁the ▁Empire ' s ▁second ▁largest ▁city , ▁it ▁was ▁quickly ▁re - capt ured ▁by ▁the ▁By z ant ines . ▁Thomas ' s ▁ 8 2 1 ▁sie ge ▁of ▁Constantin ople ▁did ▁not ▁get ▁past ▁the ▁city ▁walls , ▁and ▁he ▁was ▁forced ▁to ▁retre at . ▁ ▁The ▁Arab s ▁did ▁not ▁rel in qu ish ▁their ▁designs ▁on ▁Asia ▁Minor ▁and ▁in ▁ 8 3 8 ▁began ▁another ▁invasion , ▁s ack ing ▁the ▁city ▁of ▁Am or ion . ▁ ▁Sic ily , ▁Italy ▁and ▁Cre te ▁▁ ▁While ▁a ▁relative ▁equilibrium ▁re igned ▁in ▁the ▁East , ▁the ▁situation ▁in ▁the ▁western ▁Mediter rane an ▁was ▁irre tr iev ably ▁alter ed ▁when ▁the ▁Ag hl ab ids ▁began ▁their ▁slow ▁con quest ▁of ▁Sic ily ▁in ▁the ▁ 8 2 0 s . ▁Using ▁Tun is ia ▁as ▁their ▁launch ing ▁pad , ▁the ▁Arab s ▁started ▁by ▁conqu ering ▁Pal ermo ▁in ▁ 8 3 1 , ▁Mess ina ▁in ▁ 8 4 2 , ▁En na ▁in ▁ 8 5 9 , ▁cul min ating ▁in