Uballa on the Persian Gulf was the key of South Iraq. When Khalid bin Walid began his campaigns in Iraq he started with Uballa and occupied it without much resistance. Later as the Muslims won victories after victories in Iraq the focus shifted north west to Hira.
When Khalid bin Walid went to Syria very few Muslim forces were left in Iraq. Consequently the Muslims abandoned many posts in Iraq including Uballa. Uballa was re-occupied by the Persians. A small Muslim force under Qutba bin Qatada, however, continued to be stationed in the nighbourhood of Uballa to protect the routes to Arabia.
When Umar sent Abu Ubaid on the main Iraq front, he felt that it was necessary to send some reinforcement to the southern sector as well. Umar accordingly sent a contingent under Shareeh b. Amr to reinforce Qutba. Qutba was instructed to raid deeper into Persia.
Qutba sent Shareeh across the Tigris to raid Ahwaz. In the way at Daris, Shareeh was intercepted by the Persian forces and killed.
After the battle of Buwaib, Qutba wrote to Umar asking for more aid for intensifying activities in the southern sector.
Umar realised the importance of the southern sector. He sent for Utba b Ghazwan an early Companion and offered him the command of the southern sector. Addressing him the Caliph said:
"Allah Most High and Mighty has given Hira and what is around it to your brothers who have subdued the region of Babylon. Many of the Persian nobles have been killed. I feel that the Persians from the south will go to the help of the Persians in the north west. My strategy is to prevent the Persians on one side from helping the Persians on the other. Go to the region of Uballa and keep the people of Ahwaz and Fars and Meisan occupied so that they do not help their comrades in the Suwad.
Fight them in the hope that Allah will give you victory. March with faith in Allah and fear Allah. Be fair in judgement; say your prayers at the appointed times and remember Allah much."
Utba bin Ghazwan set off from Madina with 2,000 men and arrived in the neighbourhood of Uballa in June 635. He took over the command of the sector. The Muslims were encamped at a site twelve miles from modern Basra amidst the ruins of an ancient town.
The Commander of the Persian forces of the district of Furat marched to battle. His strategy was to fall upon the, Muslims unawares and thereby crush them. When the Persian forces arrived they found the Muslims ready for war. In the battle that followed the Persians were defeated. The Muslims pursued the defeated Persians to Uballa. No resistance was offered to the Muslims at Uballa which was occupied by the Muslims in September 635 A.D.
With Uballa as the base, Utba sent a force across the Tigris which occupied the district of Furat. The Muslim forces next marched into the district of Meisan. The Persians contested the advance of the Muslims but they were defeated and the entire district of Meisan was occupied by the Muslims. Another Muslim force advanced further afield and occupied the district of Abarqubaz. Another column captured Mazar. After subjugating these areas the Muslim forces returned to Uballa. The southern sector was now under the command of the Muslims, and the Persian supply line from Fars was cut off.
A little later the Governor of Abarqubaz revolted. Utba sent a column under Mugheera b. Shu'ba to deal with the revolt. The two forces met at Marghab. The Persians were defeated, and their Commander Feelhan was killed.
Next, there was a revolt in the district of Meisan. A column under Mugheera marched against the rebels and the revolt was successfully suppressed.
By November 635 A.D. the Muslim hold in the southern sector was quite firm Utba went on a short leave to Madina, where he died. Umar appointed Mugheera b. Shu'ba to the command of the Muslim forces in South Iraq.