Battle of JalulaJalawla, Iraq
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 withdrawal from Ctesiphon, the Persian armies gathered at Jalula north-east of Ctesiphon, a place of strategic importance from where routes led to Iraq, Khurasan and Azerbaijan.
The Caliph decided to deal with Jalula first; his plan was first to clear the way north before any decisive action against Tikrit and Mosul. Some time in April 637, Hashim marched at the head of 12,000 troops from Ctesiphon and after defeating the Persians at the Battle of Jalula, laid siege to Jalula for seven months, until it surrendered on the usual terms of Jizya.