The Parthian Empire (/ˈpɑrθiən/; 247 BC – 224 AD), also known as the Arsacid Empire /ˈɑrsəsɪd/, was a major Iranian political and cultural power in ancient Iran. Its latter name comes from Arsaces I of Parthia who, as leader of the Parni tribe, founded it in the mid-3rd century BC when he conquered the region of Parthia in Iran's northeast, then a satrapy (province) in rebellion against the Seleucid Empire. Mithridates I of Parthia (r. c. 171–138 BC) greatly expanded the empire by seizing Media and Mesopotamia from the Seleucids. At its height, the Parthian Empire stretched from the northern reaches of the Euphrates, in what is now central-eastern Turkey, to eastern Iran.