The Battle of Ullais was fought between the forces of the Rashidun Caliphate and the Sassanid Persian Empire in the middle of May 633 A.D 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. Before taking on the Persians Khalid ibn al-Walid wrote to Hormuz, the Persian governor of the frontier district of Dast Meisan saying: