The Battle of Ullais was fought between the forces of the Rashidun Caliphate and the Sassanid Persian Empire in the middle of May 633 CE in Iraq, and is sometimes referred to as the Battle of Blood River since, as a result of the battle, there were enormous amounts of Sassanian and Arab Christian casualties. This was now the last of four consecutive battles that were fought between invading Muslims and the Persian army. After each battle, the Persians and their allies regrouped and fought again. These battles resulted in the retreat of the Sassanid Persian army from Iraq and its capture by Muslims under the Rashidun Caliphate.