Siege of Constantinople ( 626 ) The Siege of Constantinople in 626 by the Avars , aided by large numbers of allied Slavs and the Sassanid Persians , ended in a strategic victory for the Byzantines . The failure of the siege saved the Empire from collapse , and , combined with other victories achieved by Emperor Heraclius ( r . 610 – 641 ) the previous year and in 627 , enabled Byzantium to regain her territories and end the destructive Roman - Persian Wars by enforcing a favorable treaty with borders " status quo " circa 590 . Background . In 602 , Phocas overthrew Maurice ( r . 582 – 602 ) , the incumbent Byzantine emperor , and established a reign of terror and incompetence , leading the Empire into anarchy . Laws were passed condemning Jews whilst religious and administrative mishandling left the Empire in a sorry state when the Sassanid king Khosrau II ( r . 590 – 628 ) attacked , using the coup as a pretext for war . Initially , the war went well for the Persians , until only Anatolia remained in Roman hands . Later , Phocas was overthrown by the son of the then Exarch of Carthage , Heraclius . A general of astounding energy yet limited experience , Heraclius immediately began undoing much of Phocas ' s damaging work that he had procured whilst Emperor . Yet , despite his offensives into Mesopotamia ( modern - day Iraq ) Heraclius was unable to stop his Persian enemies from laying siege to his capital where from Chalcedon they were able to launch their attack . From 14 – 15 May 626 , riots in Constantinople against John Seismos occurred because he wanted to cancel the bread rations of the scholae or imperial guards and raise the cost of bread from 3 to 8 follis . He did this to conserve government resources , but he was removed . However , there were further disturbances in the city . Siege . Also assisting in the siege was a host of 80 , 000 Avars , bent on removing all Roman / Byzantine imperial rule over Europe . The Persians had arrived in Chalcedon before Phocas was overthrown . However , it was only when the Avars began moving forward heavy siege equipment towards the Theodosian Walls that a siege became clear . Fortunately for the defenders , the soldiers of the capital numbered some 12 , 000 and consisted of cavalry - normally a well - trained arm of the Greco - Roman army at the time . Adding no small bonus was the Patriarch of Constantinople whose cries for religious zeal among the peasantry around Constantinople was made ever more effective by the fact that they were facing heathens . Consequently , every assault became a doomed effort . When the Avar fleet and the Persian fleet were sunk in two different naval engagements , the attackers panicked and fled abandoning the siege apparently under the belief that divine intervention had won the day for Byzantium . Aftermath . The loss came just after news had reached them of yet another Byzantine victory , where Heraclius ' s brother Theodore scored well against the Persian general Shahin . Following this , Heraclius led an invasion into Mesopotamia once again , defeating another Persian army at Nineveh . Afterwards , he marched on to Ctesiphon where anarchy reigned allowing Heraclius to extract ever more favorable terms as one Persian king was overthrown by another . Eventually , the Persians were obliged to withdraw all armed forces and return Egypt , the Levant and whatever imperial territories of Mesopotamia and Armenia were in Roman hands at the time of an earlier peace treaty in circa 595 . The war over , neither the Persians nor the Byzantines would cross swords again until the Arab - Islamic invasion broke the power of both empires . Assessment . The siege of 626 failed because the Avars did not have the patience or technology to conquer the city . The walls of Constantinople easily defended against the siege towers and engines . Furthermore , the Persians and Slavs did not have a strong enough navy to ignore the sea walls and establish a channel of communication . The lack of supplies for the Avars eventually caused them to abandon the siege .