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."