Khuzestan Central Persia Caucasus Pars Khorasan Other geographies Ridda Wars Conquest of Sasanian Persia Conquest of Byzantine Syria Campaigns in Africa Campaigns in Armenia and Anatolia The Battle of Ullais (Arabic: معركة أليس) was fought between the forces of the Rashidun Caliphate and the Sasanian Persian Empire in the middle of June 633 AD 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 Persian Sasanian and Arab Christian casualties.

These battles resulted in the retreat of the Sasanian Persian army from Iraq and its capture by Muslims under the Rashidun Caliphate.

[9] As Jaban set off with the army, Bahman Jaduya returned to Ctesiphon to discuss certain matters with the Emperor.

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.

In order to deny the Persians time to organize and to coordinate their plans, Khalid decided to fight the battle that very same day.

It was mealtime[13] and the Persian soldiers were to take their meal, but the Sasanian troops abstained from food so as to "display their toughness" to the Muslims.

[14] 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.

The Islamic historian Tabari states that upon being unable to make a breakthrough the defensive lines of the Sassanians, Khalid had prayed to Allah and promised that he would flow rivers of blood of the Sassanid forces if he won the battle.

[15][1] Khalid Ibn Walid ordered some of his men to publicly announce that the defeated Persians were to be captured, not killed--unless they fought back.

After the battle, Khalid imposed the jizyah (tax on non-muslims) on the civilians of Ullais and mandated them to act as spies among the Iranians.

The inhabitants were given peace on the terms of annual payment of jizya and agreed to provide intelligence for Muslims.

In 634 AD Abu Bakr ordered him to proceed to Syria with half of his army to command the invasion of the Byzantine Empire.

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.