<P> Jerusalem was an important city of the Byzantine province of Palestina Prima . Just 23 years prior to the Muslim conquest, in 614, it fell to an invading Sassanid army under Shahrbaraz during the last of the Byzantine - Sassanid Wars . The Persians looted the city, and are said to have massacred its 90,000 Christian inhabitants . As part of the looting, the Church of the Holy Sepulchre was destroyed and the True Cross captured and taken to Ctesiphon as a battle - captured holy relic . The Cross was later returned to Jerusalem by Emperor Heraclius after his final victory against the Persians in 628 . It was believed that the Jews, who were persecuted in their Roman - controlled homeland, had aided the Persians . </P> <P> After the death of Muhammad in 632, Muslim leadership passed to Caliph Abu Bakr following a series of campaigns known as the Ridda Wars . Once Bakr's sovereignty over Arabia had been secured, he initiated a war of conquest in the east by invading Iraq, then a province of the Sassanid Persian Empire; while on the western front, his armies invaded the Byzantine Empire . </P> <P> In 634, Abu Bakr died and was succeeded by Umar, who continued his own war of conquest . In May 636, Emperor Heraclius launched a major expedition to regain the lost territory, but his army was defeated decisively at the Battle of Yarmouk in August 636 . Thereafter, Abu Ubaidah, the Muslim commander - in - chief of the Rashidun army in Syria, held a council of war in early October 636 to discuss future plans . Opinions of objectives varied between the coastal city of Caesarea and Jerusalem . Abu Ubaidah could see the importance of both these cities, which had resisted all Muslim attempts at capture . Unable to decide on the matter, he wrote to Caliph Umar for instructions . In his reply, the caliph ordered them to capture the latter . Accordingly, Abu Ubaidah marched towards Jerusalem from Jabiya, with Khalid ibn Walid and his mobile guard leading the advance . The Muslims arrived at Jerusalem around early November, and the Byzantine garrison withdrew into the fortified city . </P> <P> Jerusalem had been well - fortified after Heraclius recaptured it from the Persians . After the Byzantine defeat at Yarmouk, the Patriarch of Jerusalem Sophronius repaired its defenses . The Muslims had so far not attempted any siege of the city . However, since 634, Saracen forces had the potential to threaten all routes to the city . Although it was not encircled, it had been in a state of siege since the Muslims captured the neighboring forts of Pella and Bosra . After the Battle of Yarmouk, the city was severed from the rest of Syria, and was presumably being prepared for a siege that seemed inevitable . When the Muslim army reached Jericho, Sophronius collected all the holy relics including the True Cross, and secretly sent them to the coast, to be taken to Constantinople . The Muslim troops besieged the city some time in November 636 . Instead of relentless assaults on the city, they decided to press on with the siege until the Byzantines ran short of supplies and a bloodless surrender could be negotiated . </P>

Who led the muslim forces that retook jerusalem