Iran, officially the Islamic Republic of Iran, and also known as Persia, is a country in West Asia. It borders Iraq to the west, Turkey, Azerbaijan, and Armenia to the northwest, the Caspian Sea to the north, Turkmenistan to the northeast, Afghanistan to the east, Pakistan to the southeast, and the Gulf of Oman and the Persian Gulf to the south. With a population of over 92 million, Iran ranks 17th globally in both geographic size and population and is the sixth-largest country in Asia. It is divided into five regions with 31 provinces. Tehran is the nation's capital, largest city, and financial center. Home to one of the world's oldest continuous major civilizations, most of Iran was first united as a nation by the Medes under Cyaxares in the 7th century BC and reached its territorial height in the 6th century BC, when Cyrus the Great founded the Achaemenid Empire. Alexander the Great conquered the empire in the 4th century BC. An Iranian rebellion in the 3rd century BC established the Parthian Empire, which later liberated the country. In the 3rd century AD, the Parthians were succeeded by the Sasanian Empire, which oversaw a golden age in the history of Iranian civilization. During this period, ancient Iran saw some of the earliest developments of writing, agriculture, urbanization, religion, and administration. Once a center for Zoroastrianism, Iran underwent Islamization following the 7th century Muslim conquest.