Arsacid Dynasty
In 126 B.C., the Parthians, a nomadic Iranian people lead 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.