Iraq occupies the alluvial plains between the Tigris and Euphrates rivers in a region called Mesopotamia. The population stood at approximately 44 million in 2023 census projections. Arabs constitute roughly 75 to 80 percent of the population, Kurds 15 to 20 percent, with smaller populations of Turkmen, Assyrians, Armenians, and other groups. Arabic and Kurdish hold official language status under the 2005 constitution. Roughly 95 to 98 percent of Iraqis are Muslim, with Shia Muslims forming 60 to 65 percent of the total and Sunni Muslims 32 to 37 percent. Christian communities, primarily Chaldean Catholic and Assyrian, numbered between 200,000 and 500,000 as of 2020, down from approximately 1.4 million in 2003. Yazidis, concentrated in Nineveh Governorate, numbered around 550,000 before the 2014 genocide, with current estimates below 400,000. Mandaeans, followers of an ancient Gnostic religion centered in the southern marshes, declined from approximately 60,000 in 2003 to fewer than 10,000 by 2020.
Baghdad serves as capital and largest city, with a metropolitan population exceeding 8 million. Basra, the southern port city near the Persian Gulf, holds approximately 2.5 million residents. Mosul in Nineveh Governorate contained roughly 1.8 million before suffering extensive destruction during the 2014-2017 conflict with the Islamic State. Erbil, capital of the Kurdistan Region, houses around 1.5 million. Najaf and Karbala, the two holiest cities in Shia Islam, each contain approximately 1 million residents, numbers that swell during religious pilgrimages. Sulaymaniyah, the Kurdistan Region's cultural center, holds roughly 1.2 million. Kirkuk, a multi-ethnic city with significant oil reserves, contains approximately 1 million. Nasiriyah near the ancient city of Ur holds around 550,000. Ramadi and Fallujah in Anbar Governorate each contain between 200,000 and 400,000, both having sustained heavy damage during conflicts between 2004 and 2016.
Mesopotamia produced the world's first known writing system. Sumerians developed cuneiform script around 3200 BCE on clay tablets found at Uruk, an ancient city in what is now Muthanna Governorate. The Sumerian city-states, including Ur, Uruk, Lagash, and Eridu, flourished between 4500 and 1900 BCE in southern Mesopotamia. The Akkadian Empire under Sargon of Akkad, ruling from approximately 2334 to 2279 BCE, created the first known multi-ethnic empire spanning Mesopotamia. Babylon rose to dominance under Hammurabi, who ruled from approximately 1792 to 1750 BCE and issued the Code of Hammurabi, a 282-law text inscribed on a 2.25-meter basalt stele now housed in the Louvre Museum. The Assyrian Empire, centered at Ashur and later Nineveh, reached its territorial peak under Ashurbanipal, who ruled from 668 to 627 BCE and assembled a library containing over 30,000 clay tablets at Nineveh. The Neo-Babylonian Empire under Nebuchadnezzar II, ruling from 605 to 562 BCE, constructed the Ishtar Gate and the city walls of Babylon, portions of which were reconstructed during the 1980s.
The Achaemenid Persian Empire conquered Babylon in 539 BCE under Cyrus the Great. Alexander the Great took Babylon in 331 BCE and died there in 323 BCE at age 32. The Seleucid Empire controlled Mesopotamia from 312 to 141 BCE, followed by the Parthian Empire until 224 CE. The Sasanian Empire ruled from 224 to 651 CE, constructing the Taq Kasra at Ctesiphon, a vaulted hall spanning 25 meters that remains the largest single-span brick arch in the world. Arab Muslim armies defeated the Sasanians at the Battle of al-Qadisiyyah in 636 CE and at the Battle of Nahavand in 642 CE. The Rashidun Caliphate established Kufa as a garrison city in 638 CE and Basra in 636 CE. The Umayyad Caliphate ruled from Damascus from 661 to 750 CE. The Abbasid Caliphate founded Baghdad in 762 CE under Caliph al-Mansur, who designed it as a circular city with a diameter of approximately 2.5 kilometers. Baghdad became the world's largest city by 930 CE with a population estimated between 1 and 1.5 million, serving as the center of the Islamic Golden Age from roughly 800 to 1258 CE.