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.
Khalid prayed to Allah for victory. He pledged "O God, if you give us victory, I shall see that no enemy warrior is left alive until the river runs red with their blood." It was typical soldier's pledge, and it inspired Khalid and his men to greater violence and fury. That paid dividends, and against the Muslim pressure, the Persian resistance ultimately broke down. By the afternoon a greater part of the Persian and Christian Arab army had been shattered, and the battle was over. The Muslims had secured a brilliant victory against the Persians for the fourth time.
Consequences of the battle of Ulleis. As the Persian army fled from the battlefield, the Muslim cavalry galloped out in pursuit of the fugitives, who had crossed the river Kaseet, and were fleeing in the direction of Hirah. These fugitives were overtaken, disarmed, and driven back to Ulleis. As each group was brought back, it was herded to the river. They were beheaded on the riverbank, and their blood ran into the river. This process of slaughter went on for three days and so large were the killings that the river virtually became the river of blood. According to the historian Tabari, 70,000 Persian and Christian Arabs lost their lives as a result of the battle of Ulleis.
The Ulleis disaster unnerved the Persian and the Christian Arabs. The local inhabitants of the region of Ulleis entered into a pact with the Muslims whereunder they agreed to pay 'Jizya' in lieu of Muslim protection. They also undertook to act as spies and guides for the Muslims.
Of all the battles fought by Khalid, the battle of Ulleis was the toughest. At the battle of Mauta, which Khalid had fought during the lifetime of the Holy Prophet, he had to face a grave situation, but the situation at Ulleis was graver still. Khalid is on record to have said: "At Mauta I broke nine swords in my hand. But I have never met an enemy like the Persians, and among the Persians I have never met an enemy like the Persian army at Ulleis."