▁the ▁land ward ▁side , ▁isol ating ▁the ▁capital . ▁Their ▁attempt ▁to ▁complete ▁the ▁block ade ▁by ▁sea ▁however ▁failed ▁when ▁the ▁By z antine ▁n avy ▁employed ▁Greek ▁fire ▁against ▁them ; ▁the ▁Arab ▁fleet ▁kept ▁well ▁off ▁the ▁city ▁walls , ▁leaving ▁Constantin ople ' s ▁supply ▁routes ▁open . ▁For ced ▁to ▁extend ▁the ▁sie ge ▁into ▁winter , ▁the ▁bes ie ging ▁army ▁suffered ▁hor rend ous ▁cas ual ties ▁from ▁the ▁cold ▁and ▁the ▁lack ▁of ▁prov isions . ▁ ▁In ▁spring , ▁new ▁rein for c ements ▁were ▁sent ▁by ▁the ▁new ▁cal i ph , ▁U mar ▁ibn ▁Abd ▁al - Az iz ▁( r . ▁ 7 1 7 – 7 2 0 ), ▁by ▁sea ▁from ▁Africa ▁and ▁Egypt ▁and ▁over ▁land ▁through ▁Asia ▁Minor . ▁The ▁cre ws ▁of ▁the ▁new ▁fle ets ▁were ▁composed ▁mostly ▁of ▁Christians , ▁who ▁began ▁defect ing ▁in ▁large ▁numbers , ▁while ▁the ▁land ▁forces ▁were ▁amb ushed ▁and ▁defeated ▁in ▁B ith yn ia . ▁As ▁fam ine ▁and ▁an ▁ep ide mic ▁continued ▁to ▁pl ague ▁the ▁Arab ▁camp , ▁the ▁sie ge ▁was ▁abandoned ▁on ▁ 1 5 ▁August ▁ 7 1 8 . ▁On ▁its ▁return , ▁the ▁Arab ▁fleet ▁suffered ▁further ▁cas ual ties ▁to ▁storm s ▁and ▁an ▁er u ption ▁of ▁the ▁vol cano ▁of ▁Th era . ▁ ▁St abil ization ▁of ▁the ▁front ier , ▁ 7 1 8 – 8 6 3 ▁▁ ▁The ▁first ▁wave