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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, not only bolstering the Arab army but also giving them more confidence, causing them to routed defeat and forcing force 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 With the main Persian army shattered, key outcome came now became evident 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. Second, the next with no major defeat for army left to face, the Sasanians Arab army was at their capital now able to march to Ctesiphon, where their the great Sasanian capital was captured after . After a two month siege, the capital fell and the Arabs, for the first time, had conquered a major metropolis that was a truly global city, as it had trade connections that spanned almost the extent of the Old World. The loss of the city also 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>