Christian Arabs. After their defeats in the battles of Kazima, Mazar and Walaja, the Persians felt that there should be a change in their strategy. They decided to settle Christian Arabs against the Muslim Arabs. In pursuance of this policy, after the battle of Walaja, the Christian Arabs mustered at Ulleis, ten miles from Walaja in another bid to drive the Muslims from Iraq. The Persian Commander-in-Chief decided to send another Persian force to Ulleis to reinforce the Christian Arabs. This force was commanded by Jaban.

Khalid's march to Ulleis. The strategy of Khalid was to pounce upon the Christian Arabs before the arrival of the army of Jaban. Khalid, therefore, rushed to Ulleis to meet the Christian Arabs. When Khalid reached Ulleis with his force, he found that the Persian army under Jaban had already arrived there. Khalid, thereupon decided to surprise the enemy. The Persian soldiers were having their meals when Khalid ordered his force to launch the attack. Hurriedly, Jaban deployed his forces to face the Muslims. The Persian troops were massed in the center, while the Christian Arabs led by Abdul Aswad and Abjar formed the right and left wings.

The battle of Ulleis. The battleground lay between the river Euphrates and its tributary Kaseef. The battlefront extended to about two miles. The battle began with a personal duel between Abdul Aswad, Christian Arab commander, and Khalid. The combat was evenly matched, but Khalid succeeded in killing his adversary. Thereafter the Muslims launched the attack against the Persians. The Persians stood as a rock, and showed no signs of any weakening. The Muslims renewed the charge, and the Persians offered stiff resistance. The Muslim attack did not yield the desired result, and as the Muslim attacks appeared to lose force, a counter attack from the Persians was expected. In view of the limited space, there were no possibilities of a maneuver here, and Khalid was afraid that in frontal attack, the Persians in view of their superiority of strength had the advantage and were likely to carry the day.