Hyrcania was the the name of a satrapy located in the territories of present day Golestan, Mazandaran, Gilan and part of Turkmenistan, lands south of the Caspian Sea. To the Greeks, the Caspian Sea was the "Hyrcanian Sea".
Hyrcania became part of the Persian Empire during the reign of Cyrus the Great (559-530 BC) or Cambyses (530-522 BC). Under the Achaemenids, it seems to have been administered as a sub-province of Parthia and is not named separately in the provincial lists of Darius and Xerxes. From the Behistun inscription we know that it was Persian by 522. The story is as follows: After the death of Cambyses, the Magian usurper Gaumâta, who did not belong to the Achaemenian dynasty, usurped the throne. The adherents of the Persian royal house, however, helped Darius to become king; he killed the usurper on September 29, 522 BC. Almost immediately, the subjects of the empire revolted. When Darius was suppressing these rebellions and stayed in Babylon, the Median leader Phraortes made his bid for power (December 522). His revolt soon spread to Armenia, Assyria, Parthia and Hyrcania. However the Persian garrison in Parthia still held out. It was commanded by Darius' father Hystaspes. On March 8, 521 BC, the Parthians and their allies, the Hyrcanians, attacked the Persian garrison, but they were defeated. Not much later, Darius was able to relieve his father. This was the first appearance in history of the Hyrcanians.
In the fifth century BC, the Greek researcher Herodotus of Halicarnassus mentions them several times in his Histories. He has a confused report on irrigation (3.117), which may be compared to the statement of the second-century historian Polybius that the Persians had built large irrigation works (World history 10.28.3). Herodotus also tells us that Hyrcanian soldiers were part of the large army which king Xerxes I (486-465) commanded against the Greeks in 480. The historian notes that they carried the same arms as the Persians.
It is possible -but not proven- that during the Persian period, a wall was built to defend Hyrcania against the nomads of the Central Asian steppe. The ruins of the wall north of the river Gorgân, which are still visible today, called the Gates of Alexander, were built later, but they probably replaced a Persian defense work.
In the confused years after the death of king Artaxerxes I Makrocheir (465-434), three of his sons succeeded to the throne: Xerxes II, Sogdianus and Darius II. The latter was a satrap in Hyrcania and may have used troops from Hyrcania and the 'upper satrapies' - that is Aria, Parthia, Arachosia, Bactria, and Sogdiana.
Hyrcania makes its reappearance in history when the Macedonian king Alexander the Great (336-323) invaded Asia. Hyrcanians are mentioned during the battle of Gaugamela (October 1, 331), and in August 329, when the last Persian king, Darius III Codomannus, was dead, many Persian noblemen fled to Hyrcania, where they surrendered to Alexander (a.o. Artabazus).