▁prolong ed ▁and ▁es cal ating ▁By z antine – S as an ian ▁wars ▁of ▁the ▁ 6 th ▁and ▁ 7 th ▁centuries ▁and ▁the ▁rec urr ing ▁out break s ▁of ▁b ub onic ▁pl ague ▁( P lag ue ▁of ▁Justin ian ) ▁left ▁both ▁emp ires ▁exha usted ▁and ▁vulner able ▁in ▁the ▁face ▁of ▁the ▁sudden ▁emer gence ▁and ▁expansion ▁of ▁the ▁Arab s . ▁The ▁last ▁of ▁the ▁wars ▁between ▁the ▁Roman ▁and ▁Pers ian ▁emp ires ▁ended ▁with ▁victory ▁for ▁the ▁By z ant ines : ▁Emperor ▁Her ac lius ▁reg ained ▁all ▁lost ▁territ ories , ▁and ▁restored ▁the ▁True ▁Cross ▁to ▁Jerusalem ▁in ▁ 6 2 9 . ▁ ▁Nevertheless , ▁neither ▁emp ire ▁was ▁given ▁any ▁chance ▁to ▁recover , ▁as ▁within ▁a ▁few ▁years ▁they ▁found ▁themselves ▁in ▁conflict ▁with ▁the ▁Arab s ▁( new ly ▁un ited ▁by ▁Islam ), ▁which , ▁according ▁to ▁Howard - John ston , ▁" can ▁only ▁be ▁li ken ed ▁to ▁a ▁human ▁t sun ami ". ▁According ▁to ▁George ▁L iska , ▁the ▁" un necess arily ▁prolong ed ▁By z antine – Pers ian ▁conflict ▁opened ▁the ▁way ▁for ▁Islam ". ▁ ▁In ▁late ▁ 6 2 0 s , ▁the ▁Islam ic ▁Pro ph et ▁Muhammad ▁had ▁already ▁managed ▁to ▁un ify ▁much ▁of ▁Arab ia ▁under ▁Muslim ▁rule ▁via ▁con quest ▁as ▁well ▁as ▁making ▁al li ances ▁with ▁neighbor ing ▁tribes , ▁and ▁it ▁was ▁under ▁his ▁leadership ▁that