after Alexander's death, his generals fought for and divided up his empire. Parthian Empire In 126 B.C., the Parthians, a nomadic Iranian people led by the Arsacid Dynasty, captured the Tigris–Euphrates river valley. The Parthians were able to control all trade between the East and the Greco–Roman world. For the most part, they chose to retain existing social institutions and to live in cities that already existed. Mesopotamia was immeasurably enriched by this, the mildest of all foreign occupations of the region. The population of Mesopotamia was enormously enlarged, Iranians, and Aramaeans. With the exception of the Roman occupation under Trajan (A.D. 98–117) and Septimius Severus (A.D. 193–211), the Arsacids ruled until a new force of native Iranian rulers, the Persian Sassanids, conquered the region in A.D. 227. Sassanid Iranian Empire During the time of the Sassanid Empire, from the 3rd century to the 7th century, the major part of Iraq was called in Persian Del-e Īrānshahr (lit. "the heart of Iran"), and its metropolis Ctesiphon (not far from present-day Baghdad) functioned for more than 800 years as the capital city of Iran. Medieval era Abbasid Caliphate The Abbasid Caliphate of circa 650 A.D. was the second of the two great Islamic caliphates. It was ruled by the Abbasid dynasty of caliphs, who built their capital in Baghdad (Iraq). The Abbasids had depended heavily on the support of Persians in their overthrow of the Umayyads. Abu al-'Abbas' successor, al-Mansur, moved their capital from Damascus to the new city of Baghdad and welcomed non-Arab Muslims to their court. During the reign of its first seven caliphs, Baghdad became a center of power where Arab and Iranian cultures mingled to produce a blaze of philosophical, scientific, and literary glory. This era is remembered throughout the Muslim world, and by Iraqis in particular, as the pinnacle of the Islamic past. Abbasid Iraq eventually became part of Buyid Iran as the Buyid Emirate of Baghdad. After the Seljuk invasion, the Abbasid Caliphate regained a degree of independence. During the reign of Caliph Al-Nasir, Abbasid Iraq expanded into Iran. Late Middle Ages Iran and Iraq both fell to the Mongols during the 13th century. In the 14th century, there emerged the Mongol Jalayirid sultanate which controlled Iraq and parts of Iran. They were later displaced by the Timurids, Qara Qoyunlu, and then the Aq Qoyunlu of Iran, however the area centered around Basra was always