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In 636 CE, there were two major universal religions, which were Christianity and Zoroastrianism. These faiths were supported by the two major superpowers of the Middle East, which were the Byzanitine and Sasanian Empires. Both these powers seemed invincible and held much great wealth. However, by the early 7th century CE, major wars across the Middle East devastated much of the region and led to both these empires to deplete their resources.<ref>For more on background to this period, see: Cutler, Anthony. 2009. <i>Image Making in Byzantium, Sasanian Persia, and the Early Muslim World.</i> Variorum Collected Studies Series. Farnham, Surrey, England ; Burlington, VT: Ashgate/Variorum.</ref>

In 636, Caliph Umar had re-invaded Iraq, after a previous Muslim army was defeated and removed from the region. The Sasanian and Arab armies met at the village of Qadisiyyah, southwest of the modern city of Hilla in Iraq, which is near ancient Babylon. The battle was mostly fought by a relatively inexperienced Arab army; however, much of the Persian army was also inexperienced, as many of their best troops had died or were depleted from previous engagements. This made the two sides relatively even and after the first day of the battle it did not seem that either of them could break each other's defenses. A key turning point was Arab reinforcements that strengthened the Islamic army, who were much more professional soldiers and well trained, and that arrived at the battle of Qadisiyyah on the second day. These numbered about 5000 and proved to be the difference in allowing the Arabs to breakthrough the Persian main line, causing them to routed and forcing the main Persian force to retreat. By the fourth day of the battle, the outcome was clear and the Sasnian army fled to the north, although many were killed as they attempted to flee.<ref>For more on the battle, see: <Madelung, Wilferd. 1998. <i>The Succession to Muhammad: A Study of the Early Caliphate.</i> Cambridge: Cambridge University Press.</ref>

The key outcome came after the battle. First, a large amount of booty was captured from the Sasanian army. This helped finance later battles and the Muslim army to further march north. The Second, the next major defeat for the Sasanians was at their capital Ctesiphon, where their capital was captured after a two month siege. The loss of the city led to the full conquest of Mesopotamia and Khuzestan, which were the most important provinces in the Sasanian Empire. In 650, a new phase of invasions was then ready, leading to the conquest of Persia and defeat of the Sasanian Empire.<ref>For more on the conquest of the Middle East by the Arab armies and invasion and conquest of Iran, see: Savant, Sarah Bowen. 2013. <i>The New Muslims of Post-Conquest Iran: Tradition, Memory and Conversion.</i> Cambridge Studies in Islamic Civilization. Cambridge ; New York: Cambridge University Press.</ref>