The Islamic conquest of Persia (637-651) led to the end of the Sassanid Empire and the eventual decline of the Zoroastrian religion in Persia. However, the achievements of the previous Persian civilizations were not lost, but were to a great extent absorbed by the new Islamic polity. Islam has been the official religion of Iran since then, except short duration after Mongol raid and establishment of Ilkhanate. Iran became an Islamic republic after the Islamic Revolution of 1979.