the Byzantines. Meanwhile, he ordered Saad to enter into peace negotiations with Yazdegerd III and invite him to convert to Islam to prevent Persian forces from taking the field. Heraclius instructed his general Vahan not to engage in battle with the Muslims before receiving explicit orders. Fearing more Arab reinforcements, Vahan attacked the Muslim army in the Battle of Yarmouk in August 636, and was routed. With the Byzantine threat ended, the Sasanian Empire was still a formidable power with vast manpower reserves, and the Arabs soon found themselves confronting a huge Persian army with troops drawn from every corner of the empire, including war elephants, and commanded by its foremost generals. Within three months, Saad defeated the Persian army in the Battle of al-Qādisiyyah, effectively ending Sasanian rule west of Persia proper. This victory is largely regarded as a decisive turning point in Islam's growth: with the bulk of Persian forces defeated, Saad with his companions later conquered Babylon (Battle of Babylon (636)), Kūthā, Sābāṭ (Valashabad) and Bahurasīr (Veh-Ardashir). Ctesiphon, the capital of the Sassanid Empire, fell in March 637 after a siege of three months. Conquest of Mesopotamia (636–638) In December 636, Umar ordered Utbah ibn Ghazwan to head south to capture al-Ubulla (known as "port of Apologos" in the Periplus of the Erythraean Sea) and Basra, in order to cut ties between the Persian garrison there and Ctesiphon. Utbah ibn Ghazwan arrived in April 637, and captured the region. The Persians withdrew to the Maysan region, which the Muslims seized later as well. After the conquest of Ctesiphon, several detachments were immediately sent west to capture Circesium and Heet, both forts at the Byzantine border. Several fortified Persian armies were still active north-east of Ctesiphon at Jalawla and north of the Tigris at Tikrit and Mosul. After withdrawal from Ctesiphon, the Persian armies gathered at Jalawla, a place of strategic importance due to routes leading from here to Mesopotamia, Khurasan and Azerbaijan. The Persian forces at Jalawla were commanded by Mihran. His deputy was Farrukhzad, a brother of Rustam, who had commanded the Persian forces at the Battle of al-Qadisiyyah. Umar decided to deal with Jalawla first, thereby clearing the way to the north, before taking any decisive action against Tikrit and Mosul. Umar appointed Hashim ibn Utbah to take Jalawla and Abdullah ibn Muta'am to conquer Tikrit and Mosul. In April 637, Hashim led 12,000 troops