Battle of Ullais 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 : 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 . Background . After defeat at the Battle of Walaja , Christian Arab survivors of the battle fled from the battlefield , crossed the River Khaseef ( a tributary of the Euphrates ) and moved between it and the Euphrates . Their flight ended at Ullais , about 10 miles from the location of the Battle of Walaja . The Muslims were aware of the presence of hostile Arabs at Ullais but , as they were less numerous and were survivors of Walaja , they never considered them a military threat until they started to regroup and the Muslim commander Khalid ibn Walid was informed about the arrival of more Arab hordes , mainly from the Christian Arab tribe of Bani Bakr . More reinforcements were raised from the Christian Arab tribes in the region between Al - Hirah and Ullais . The Rashidun Caliphate army under Khalid crossed the river Khaseef and approached Ullais frontally . Emperor Ardsheer meanwhile sent orders to Bahman Jaduya to proceed to Ullais and take command of Arab contingents there and stop the Muslims advance at Ullais . Bahman sent his senior general Jaban with the imperial army to Ullais with orders to avoid battle until Bahman Jaduya himself arrived . As Jaban set off with the army , Bahman Jaduya returned to Ctesiphon to discuss certain matters with the Emperor . He arrived at Ctesiphon to find Emperor Ardsheer very ill and remained in attendance on him . By now the Persians and Arabs had realised that the Muslims ' objective was Al - Hirah . They decided to fight and defeat the Muslims army . The Christian Arab contingents were under the command of a tribal chief called " Abdul - Aswad " , who had lost his two sons in the Battle of Walaja against the Muslims and wanted revenge . The battle . One of the Muslim commanders , Al - Muthanna ibn Haritha , advanced with the light cavalry scouts to the Ullais and informed the Muslim commander in chief Khalid ibn Walid of the location of the hostile Arabs . Khalid tried to reach Ullais before the Sassanid army could reinforce them , in order to avoid a battle with an army that would heavily outnumbered his own ; however he failed to do so . In order to deny the Persians time to organize and to coordinate their plans , Khalid decided to fight the battle that very same day . According to modern geography the battlefield lies 25 miles south - east of the Iraqi city of Najaf , and about 4 miles south - west of modern " Ash Sinafiyah " . The Sassanid army and Christian Arab s contingents were camped side by side with the Euphrates to their left , the Khaseef to their right and the river junction behind them . Muslim commander in chief Khalid ibn Walid arrayed his army in battle formation , appointing Adi ibn Hatim ( who was a son of a the Famous Arab Christian Leader Hatim At Tai and a former Christian ) as commander of the right wing and Asim ibn umar commander of the left wing . Information of the Rashidun Caliphate army ' s advance reached Jaban a little before midday . It was mealtime and the Persian soldiers were to take their meal , but the Sassanian troops abstained from food so as to " display their toughness " to the Muslims . Jaban arranged the Sassanid army in great haste before the Muslims could arrive , appointing the Christian Arab s to form the wings of his army , with the tribal chief " Abdul - Aswad " commanding the right wing and the tribal chief " Abjar " commanding the left wing . The center was formed by the Imperial army . The battlefield ran south - east of Ullais between the Euphrates and Khaseef . The Persian army was deployed with its back to Ullais , while in front of it was arrayed the Rashidun Caliphate army . The northern flank of both armies rested on the Euphrates and their southern flank on the river " Khaseef " , a distance of about 2 miles . Details of the manoeuvres used by Khalid are not recorded by history . Muslim commander in chief Khalid ibn al - Walid killed the Christian Arab tribal chief Abdul - Aswad in a duel . The fighting was heaviest on the bank of the Khaseef . It is mentioned in Muslim chronicles that " If ever an army meant to fight it out to the last , it was the imperial army of Ullais " . The fierce battle continued for hours ; no signs of weakness were shown on either side . Early in the afternoon the Sassanid Persian army and Arab allies , unable to withstand the veteran Muslim army , finally retreated to the north - west in the direction of Al - Hirah . Against the Persians and their allies , Khalid ibn al - Walid had always been heavily out numbered but understood the concept of desert warfare and when threatened , his men , who were used to the harsh desert conditions , would withdraw into the desert , where they could not be pursued . The Arab camels , drank water less frequently than the Persian horses . Khalid also used the desert for his camel supply line . Arab tribes were the only ones who could interfere with his strategy and defeat him from the rear by disrupting his supply line and stopping his escape routes . Khalid ibn al - Walid feared that these Arab tribes would regroup , others will also get bribed by the Persians and attack his supply line and close off his escape routes . He also feared that the Persians would regroup and attack his front . They had already fought him in three battles and after each battle regrouped . The Persians had a grand empire for 12 centuries and had lost battles before and risen again . After the defeat of the Persians Empire by Khalid , 300 years later many stories were invented in Persia against Khalid to oppose the cultural invasion , that found their way into Tabari ' s books . Stories that do not exist in earlier books . Tabari also invested numbers and stories to make his book into a best seller . In the modern best seller book , Shadow of the sword , The Battle for Global Empire and the End of the Ancient World , Tom Holland writes Muslim historians not surprisingly would take an exultant delight in emphasizing the immensity of the manpower available to the toppled empire : of how their troops numbered in the hundreds of thousands and how their blood when it was spilled , had flowed in such prodigious quantities as to turn the wheels of local water mills . Contemporaries knew better , though . They understood the true condition of anaemia that had afflicted both superpowers . " It is said by a Persian Muslim chronicles Tabari 300 years later that : Khalid ibn al - Walid launched his cavalry after them , with orders to capture them . The pursuit by the Muslim cavalry , the capture and the return of the Persian and Arab warriors in small groups , and their killing in the river Khaseef , which was actually a small canal used to power a water mill . Then on the advice of Qa ' qa ibn Amr , one of the commanders of the Muslim army , Khalid ordered the dam on the river to be opened . The water then flowed in . These statements do not appear in any of the older books . Many history books on these events by al - Imam al - Waqidi , Ibn Hisham and Al Baladuri and were written before al Tabri . After the battle of Battle of Ullais Khalid ibn al - Walid besieged the city of Al - Hirah in May 633 . Khalid ibn al - Walid also had a generous side and spared the city and made a deal with them . The inhabitants remained Christian and joined the Islamic State . Khalid ibn al - Walid had the flexibility of mind to adapt to the course that would best further his goal . Modern military theorists consider this the maintenance of the objective . In September 633 Khalid ibn al - Walid made an agreement with Erbil and Erbil also joined the Islamic State . Khalid ibn al - Walid was generous when dealing with cities and civilians but ruthless with nomads who fought him with armed combat from behind and tried to cut off his supply lines and escape routes . After the battle , a fine tribute was given by Khalid to the Sassanid Persian army . He said : Later in December 633 some Arab tribes assisted by a Persian garrison did exactly what Khalid ibn al - Walid had feared , in Ain al - Tamr . They attacked his supply line so Khalid ibn al - Walid fought them . Khalid ibn Walid then moved to the siege of Ein - al - Tamr and made a pact with them too . With the fall of the main cities the whole of southern and central Iraq came under Muslim control . In 634 A . D Abu Bakr ordered Khalid ibn Walid to proceed to Syria with half of his army to command the invasion of the Byzantine Empire . Misna bin Haris was left as the successor of Khalid . The Persians , under their new emperor Yazdgerd III , regrouped , concentrated new armies and defeated the Muslims in the Battle of the Bridge , and recaptured Iraq . The second invasion of Iraq was undertaken under Sa ` d ibn Abī Waqqās who , after defeating the Sassanid army at the Battle of al - Qādisiyyah in 636 AD , captured Ctesiphon . This was followed by the whole scale invasion of the Sassanid Persian Empire .