The earliest entity considered a part of the Persian Empire is Persia's
Achaemenid Empire (648�330 BC).
Successive states in Greater Iran prior to March, 1935 are collectively called
the Persian Empire by Western historians.
After Alexander's conquest and reign, subsequent rising empires in Persia derived from within the cultural mix spread by the Hellenistic conquest, and virtually all the successor empires were major regional and some major international powers in their day.
The earliest known record of the Persians comes from an Assyrian inscription
from c. 844 BC that calls them the Parsu (Parsuash, Parsumash) and mentions them
in the region of Lake Urmia alongside another group, the Madai (Medes). For the
next two centuries, the Persians and Medes were at times tributary to the
Assyrians. The region of Parsuash was annexed by Sargon of Assyria around 719
BC. Eventually the Medes came to rule an independent Median Empire, and the
Persians were subject to them.
The Achaemenids were the first to create a centralized state in Persia, founded
by Achaemenes (Hakhamanish), chieftain of the Persians around 700 BC.
Around 653 BC, the Medes came under the domination of the Scythians, and Teispes,
the son of Achaemenes, seems to have led the nomadic Persians to settle in
southern Iran around this time. The kingdom of Anshan and its successors
continued to use Elamite as an official language for quite some time after this,
although the new dynasts spoke Persian, an Indo-Iranian tongue.
According to the official story, the Achaemenid or Persian empire was founded
by Cyrus the Great, who became king of Persis in 559 BCE and defeated his
overlord Astyages of Media in 550. The size of the Median empire is not exactly
known, but it seems to have included Cappadocia and Armenia in the west and
Parthia, Aria and Hyrcania in the east. Cyrus added Lydia (547), Bactria and
Sogdia, campaigned in India, and captured the city of Babylon in 539. His
capital was Pasargadae, built on the site where he had defeated Astyages. In
530, Cyrus was killed during a campaign against the Massagetae, a Scythian
tribe.
He was succeeded by his son Cambyses, who conquered Egypt (525). Three years
later, civil war broke out when his courtier Gaum�ta revolted. Cambyses returned
home but died in Syria. A distant relative of Cambyses, the Achaemenid prince
Darius, however, killed Gaum�ta. After the second coup in one year, many
provinces of the Achaemenid empire revolted; the most important rebellions were
those of Phraortes of Media and Nidintu-B�l of Babylonia. After nineteen
battles, tranquillity returned to the Achaemenid empire. Darius described his
victory in the Behistun inscription, in which he presents himself as the
faithful servant of the Persian supreme god Ahuramazda. (We do not know whether
the Achaemenids adhered to the teachings of the Bactrian prophet Zarathustra,
although later Persian dynasties certainly were Zoroastrians.)
It may have been during the Achaemenid period that Zoroastrianism reached
South-Western Iran, where it came to be accepted by the rulers and through them
became a defining element of Persian culture. The religion was not only
accompanied by a formalization of the concepts and divinities of the traditional
(Indo-)Iranian pantheon but also introduced several novel ideas, including that
of free will, which is arguably Zoroaster's greatest contribution to religious
philosophy. Under the patronage of the Achaemenid kings, and later as the
de-facto religion of the state, Zoroastrianism would reach all corners of the
empire. In turn, Zoroastrianism would be subject to the first sycretic
influences, in particular from the Semitic lands to the west, from which the
divinities of the religion would gain astral and planetary aspects and from
where the temple cult originates. It was also during the Achaemenid era that the
sacerdotal Magi would exert their influence on the religion, introducing many of
the practices that are today identified as typically Zoroastrian, but also
introducing doctrinal modifications that are today considered to be revocations
of the original teachings of the prophet.