Iran is heir to one of the world's oldest civilizations,[18][19] beginning with the formation of the Proto-Elamite and Elamite kingdoms in 3200–2800 BC. The area was first unified by the Iranian Medes 625 BC, who became the dominant cultural and political power in the region.[4] Iran reached its greatest geographic extent during the Achaemenid Empire founded by Cyrus the Great in 550 BC, which at one time stretched from parts of Eastern Europe in the west, to the Indus Valley in the east, making it the largest empire the world had yet seen.[20] The empire collapsed in 330 BC following the conquests of Alexander the Great, but reemerged shortly after as the Parthian Empire. Under the Sassanid Dynasty, Iran again became one of the leading powers in the world for the next four centuries.[21][22]
