Siege of Nicaea ( 727 ) The Siege of Nicaea of 727 was an unsuccessful attempt by the Umayyad Caliphate to capture the Byzantine city of Nicaea , the capital of the Opsician Theme . Ever since its failure to capture the Byzantine Empire ' s capital , Constantinople , in 717 – 718 , the Caliphate had launched a series of raids into Byzantine Asia Minor . In 727 , the Arab army , led by one of the Caliph ' s sons , penetrated deep into Asia Minor , sacked two Byzantine fortresses and in late July arrived before Nicaea . Despite constant attacks for 40 days , the city held firm and the Arabs withdrew and returned to the Caliphate . The successful repulsion of the attack was a major boost for Byzantine emperor Leo III the Isaurian ' s recently initiated campaign to abolish the veneration of icon s in the Empire ; Leo claimed it as evidence of divine favour for his policy . The siege of Nicaea marks also the high point of the Umayyad raids , as new threats and defeats on their far - flung frontiers diverted Umayyad strength elsewhere , while Byzantine power gradually recovered . Background . Following the failure of the year - long assault by the Umayyad armies on the Byzantine capital Constantinople in 717 – 718 , a short period of peace followed as the Umayyads licked their wounds , suppressed the rebellion of Yazid ibn al - Muhallab and re - assessed their priorities . When warfare on the Arab – Byzantine frontier recommenced in 720 , the strategic focus of the Caliphate had shifted away from outright conquest . The Muslim raids across the Taurus Mountains into Byzantine Asia Minor still occurred regularly every spring and summer , sometimes accompanied by naval raids and followed by a winter expedition ; they devastated large tracts of Asia Minor , and destroyed several fortresses ; but the Arabs did not attempt to hold on to captured strongholds on the west side of the Taurus Mountains . Byzantine reaction during these years was passive , as the Empire still nursed its strength against the vastly superior resources of the Caliphate . The Byzantines did not obstruct or confront the raiding Arab armies , but rather retreated to well - fortified positions scattered throughout Asia Minor . After the accession of Caliph Hisham ( r . 723 – 743 ) , the scale and ambition of the Muslim raids grew . One of the most prominent Umayyad leaders in these campaigns was Hisham ' s son Mu ' awiya , who led expeditions in 725 and 726 , the first of which went as far west as Dorylaion . Invasion of 727 and the siege of Nicaea . In summer 727 , another large - scale invasion was led by Mu ' awiya , with Abdallah al - Battal heading the vanguard of the army . The Byzantine chronicler Theophanes the Confessor claims that the vanguard alone numbered 15 , 000 men and the entire invasion force 100 , 000 , clearly a grossly inflated number . Theophanes also records a certain Amr as Mu ' awiya ' s second - in - command , but Arab sources are clear in this regard . The Arab army moved west into northwestern Asia Minor , and the vanguard under al - Battal attacked and sacked the town of Gangra in Paphlagonia and a place called in Arab sources Tabya ( possibly the fort of Ateous in Phrygia ) . Gangra was razed to the ground , but during the attack on Tabya the Arabs , especially the Antiochene contingent , are said to have suffered heavy losses . From there , the Arabs turned west towards Nicaea , the chief city of Bithynia and capital of the powerful Opsician Theme . The Arabs arrived before the city in late July , with al - Battal ' s vanguard preceding the main army . The Byzantines , probably under the command of the Count of the Opsicians , Artabasdos , did not meet them in the field , but instead retreated behind the city ' s walls . The Arabs assaulted the city for forty days , employing siege engines which destroyed a part of the walls , but eventually failed to take it . In late August , they raised the siege and departed , taking along many captives and much booty . The 12th - century chronicle of Michael the Syrian claims that the city ' s inhabitants abandoned it and fled by ship through Lake Ascanius , whereupon the Arabs destroyed Nicaea , but this is clearly an error . Aftermath . The repulsion of the Arab assault on Nicaea was an important success for the Byzantines . Emperor Leo III the Isaurian ( r . 717 – 741 ) regarded the city ' s survival as a sign of divine favour towards his newly instituted iconoclastic policies , and was encouraged to drive them further . This is probably related to an incident mentioned in the account of Theophanes , where a certain Constantine , who served as a groom ( " strator " ) to Artabasdos , threw a stone on an icon of the Virgin Mary and then trampled on it . The soldier was killed the next day by a catapult , a fact which Theophanes reports as evidence of divine vengeance . However , this passage shows strong signs of tampering by the fervently anti - iconoclast Theophanes , from what was probably originally a pro - iconoclast story . Militarily , the siege of Nicaea was the high - water - mark of the post - 718 Umayyad raids ; never again would Umayyad armies penetrate as deeply into Asia Minor . Increasingly thereafter the Syro - Jaziran army , that provided the manpower for the raids against Byzantium , was diverted in the hard and fruitless wars against the Khazars in the Caucasus : the Khazars inflicted a heavy defeat on the Muslims in 730 , and a Byzantine – Khazar alliance was sealed by the marriage of Leo III ' s son and heir Constantine V ( r . 741 – 775 ) with the Khazar princess Irene shortly after . Over the next few years , while Byzantine strength revived , the Muslim military situation on all fronts of the over - extended Caliphate deteriorated . Consequently , in the 730s , Arab raids were mostly limited to the immediate frontier regions and their successes became fewer . By 740 , when the Umayyads assembled the largest invasion force fielded after 718 , the Byzantines had recovered enough to inflict a heavy defeat against them at the Battle of Akroinon .