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. It was the last of four consecutive battles that were fought between invading Muslims and the Persian army. These battles resulted in the retreat of the Sassanid Persian army from Iraq and its capture by Muslims under the Rashidun Caliphate.

The Rashidun Caliphate (Arabic: الخلافة الراشدية‎ al-khilāfat ar-Rāshidīyah), (c. 632-661) comprising the first four caliphs in Islam's history, was founded after Muhammad's death in 632 (Year 10 A.H. in the Islamic calendar). At its height, the Caliphate extended from the Arabian Peninsula, to the Levant, Caucasus and North Africa in the west, to the Iranian highlands and Central Asia in the east. At its greatest extent it was one of the largest empires in history. It is also known as the Patriarchal Caliphate.

After Muhammad's death in 632, the Medinan Ansar debated which of them should succeed him in running the affairs of the Muslims while Muhammad's household was busy with his burial. Umar (a Quraish) and Abu Ubaidah ibn al-Jarrah pledged their loyalty to Abu Bakr, with the Ansar and the Quraish soon following suit. Abu Bakr thus became the first Khalifa Rasul Allah (Successor of the Messenger of God), and embarked on campaigns to propagate Islam. First, though, he would have to subdue the Arabian tribes which had gone back on their oaths of allegiance to Islam and the Islamic community. As a Khalifa or Caliph he was not a monarch and never claimed such a title nor did his three successors do so.

Abu Bakr (Abdullah ibn Abi Quhafa) (Arabic: عبد الله بن أبي قحافة, Transliteration: ʿAbd Allāh ibn Abī Quḥāfah, c. 573 CE - 634 CE) also known as Abū Bakr as-Șiddīq (Arabic: أبو بكر الصديق) was a senior companion (Sahabi) and the father-in-law of the Islamic prophet Muhammad. He ruled over the Rashidun Caliphate from 632–634 CE when he became the first Muslim Caliph following Muhammad's death. As Caliph, Abu Bakr succeeded to the political and administrative functions previously exercised by Muhammad, since the religious function and authority of prophethood ended with Muhammad's death according to Islam. He was called Al-Siddiq (The Truthful) and was known by that title among later generations of Muslims.

As a young man, Abu Bakr became a cloth merchant and he traveled extensively in Arabia and neighboring lands in the Middle East, through which he gained both wealth and experience. He eventually came to be recognized as the chief of his clan. On his return from a business trip to Yemen, he was informed that in his absence Muhammad had openly declared his prophethood. Not long after, Abu Bakr accepted Islam and was the first person outside the family of Muhammad to openly become a Muslim. He was instrumental in the conversion of many people to the Islamic faith and early in 623, Abu Bakr's daughter Aisha was married to Muhammad, strengthening the ties between the two men.