The history of Iran can be divided into the Pre-Islamic period and the Islamic period. The pre-Islamic period includes six civilizations: Elamites (4th millennium to 1st millennium B.C), Maiden (1st millennium B.C), Achaemenids (550-330 B.C), Sulucids (312-247 B.C), Parthians (247 B.C-224 A.D), and Sassanians (224-651 A.D).
In 651 A.D, Muslim Arabs conquered Iran and declined Zoroastrianism. It was the beginning of the Irano-Islamic period which lasts until today.
Iran is a cradle of human civilization. In the late 4th and early 3rd millennium, B.C. Elamite civilization flourished on the lowland Khuzestan, in the South-west of Iran. In the late 2nd millennium B.C. the Iranians migration southward from the Central Asian Steppes Iranian plateau began. By the mid-9th century B.C. two major groups of Iranians have been dominant forces on the plateau: the Medes and the Persians.
In 550 B.C., the Persians defeated the Medes, and the Achaemenid kings appeared on the international scene. The Parthian and Sassanian dynasties ruled Iran successively. In 625 the Muslim armies defeated the Sassanians and the majority of Iranians converted to Islam, attracted by the divine teaching of this religion.
After the Arab invasion of Iran, the Tahirids were the first independent Iranian Muslim dynasty established in Iran (820 A.D). The Samanids and the Buyids also made great attempts to bring to fruition the Iranian Renaissance. In 1216 the Mongols, sweeping in with a relentless fury, obliterated many Iranian cities.
In the Safavid period (1501-1736) when Iran gained a powerful dynasty, the country made great progress. The Afsharids and Zands were the next dynasties that ruled Iran successively in the 18th century. The 19th century, when the Qajars reigned over Iran, was a time of disintegration for the country. Many events, both internal and external, caused it to fall into decay. In 1925, after the deposition of the last Qajar shah, Reza Khan transferred sovereignty to himself.
Under the reign of Reza Shah and his son, Mohammad Reza Pahlavi, the increasing political and economic dependence on western countries as well as disrespect to religious values raised hatred of people for the Pahlavi regime. This hatred was manifested in demonstrations against the government under the leadership of Ayatollah Khomeini in 1978. Eventually, these demonstrations lead to the establishment of the Islamic Republic of Iran in 1979.