Persian Empire refers to any of a series of imperial dynasties centered in Persia (modern–day Iran). The first of these was the Achaemenid Empire established by Cyrus the Great in 550 BC with the conquest of Median, Lydian and Babylonian empires. It covered much of the Ancient world when it was conquered by Alexander the Great. Several later dynasties "claimed to be heirs of the Achaemenids". Persia was then ruled by the Parthian Empire which supplanted the Hellenistic Seleucid Empire, and then by the Sassanian Empire which ruled up until mid 7th century.
It is important to note that while many of these empires referred to themselves as Persian, they were often ethnically ruled by Medes, Babylonians, or Parthians. Iranian dynastic history was interrupted by the Arab Muslim conquest of Persia in 651 AD, establishing the even larger Islamic Caliphate, and later by the Mongol invasion.
The main religion of ancient Persia was the native Zoroastrianism, but after the seventh century, it was replaced by Islam.
In the modern era, a series of Islamic–notably Shiite–dynasties (e.g. Safavids and Afsharids) ruled Persia independently of the Arab/Turkic Sunni caliphates, who became Persia's classical rival, like the pagan Romans and Christian Byzantines had been before. Since 1979 and the downfall of the Pahlavi dynasty during Iranian Revolution, Persia has had a Shiah theocratic government.