0 – 4 , 0 0 0 ▁troops ▁under ▁his ▁command , ▁' Am r ▁ibn ▁al - A ' as ▁first ▁crossed ▁into ▁Egypt ▁from ▁Palest ine ▁at ▁the ▁end ▁of ▁ 6 3 9 ▁or ▁the ▁beginning ▁of ▁ 6 4 0 . ▁He ▁was ▁progress ively ▁joined ▁by ▁further ▁rein for c ements , ▁not ably ▁ 1 2 , 0 0 0 ▁soldiers ▁by ▁Al - Z ub ay r . ▁' Am r ▁first ▁bes ie ged ▁and ▁conquer ed ▁Bab yl on , ▁and ▁then ▁attacked ▁Alexand ria . ▁The ▁By z ant ines , ▁divided ▁and ▁shock ed ▁by ▁the ▁sudden ▁loss ▁of ▁so ▁much ▁territory , ▁agreed ▁to ▁give ▁up ▁the ▁city ▁by ▁September ▁ 6 4 2 . ▁The ▁fall ▁of ▁Alexand ria ▁ex ting u ished ▁By z antine ▁rule ▁in ▁Egypt , ▁and ▁allowed ▁the ▁Muslim s ▁to ▁continue ▁their ▁military ▁expansion ▁into ▁North ▁Africa ; ▁between ▁ 6 4 3 – 6 4 4 ▁' Am r ▁completed ▁the ▁con quest ▁of ▁Cy ren a ica . ▁U th man ▁succeeded ▁C ali ph ▁U mar ▁after ▁his ▁death . ▁ ▁According ▁to ▁Arab ▁histor ians , ▁the ▁local ▁Christian ▁Co pts ▁wel com ed ▁the ▁Arab s ▁just ▁as ▁the ▁Mon oph ys ites ▁did ▁in ▁Jerusalem . ▁The ▁loss ▁of ▁this ▁luc r ative ▁province ▁dep riv ed ▁the ▁By z ant ines ▁of ▁their ▁valuable ▁whe at ▁supply , ▁thereby ▁causing ▁food ▁short ages ▁throughout ▁the ▁By z