Iran is home to one of the world's oldest continuous major civilizations, with historical and urban settlements dating back to 4000 BC.[1] The Medes unified Iran as a nation and empire in 625 BC.[2][2]
The Islamic conquest of Persia (633–656) and the end of the Sassanid Empire was a turning point in Iranian history. Iran had been a monarchy ruled by a shah, or emperor, almost without interruption from 1501 until the 1979 Islamic revolution, when Iran officially became an Islamic Republic on 1 April 1979.[3][4]