Parthia (Old Persian Parthava): satrapy of the ancient Achaemenid empire, the north-east of modern Iran, also known as Khorasan.
The borders of Parthia were the Kopet Dag mountain range in the north (today the border between Iran and Turkmenistan) and the Dasht-e Kavir desert in the south. In the west was Media, in the northwest Hyrcania, in the northeast Margiana, in the east Aria. (The road from Media through Parthia to Margiana is the famous Silk road.) On the other side of the southern desert was Persia proper. The country south of the Kopet Dag is fertile and was well-irrigated in Antiquity. There were large forests.
Assyrian texts mention a country named Partakka or Partukka in the seventh century. At an unknown moment, its inhabitants were subjected by the Medes, who ruled the first Iranian empire until they were subdued by the Persian leader Cyrus the Great in 550 BCE. For the next two centuries, Parthia was part of the Achaemenid Empire.
In 522/521, after the coup d'état of the Persian king Darius I the Great, Parthia revolted against the Persians, joining the Median rebel king Phraortes. The Persian satrap of Parthia was Hystaspes, the father of the new Persian king; he managed to stand his ground against the Parthian rebels in the city Vishpauzâtish, where he repelled his enemies on March 8, 521. After Hystaspes had received as reinforcements the troops which had captured Phraortes at Rhagae (Tehran), he was able to attack the Parthians and Hyrcanians near the Parthian town Patigrabana (July 11). After his victory, Parthia was pacified again.
When Darius' son Xerxes attacked Greece in 480 BCE, the Parthian contingent was - according to the Greek researcher Herodotus of Halicarnassus - commanded by Artabazus, the son of Pharnaces, the chief economic official of the Achaemenid empire. The Athenian playwright Aeschylus tells us that among the Persian commanders who were killed during this war, was a cavalry leader "on a mail-clad horse" called Arsaces.note[Aeschylus, Persians 996.] The remark is interesting for two reasons: in the first place because the name was to be that of the future kings of the Parthian empire, and in the second place because the man is mentioned as a cavalry commander, something for which the Parthians were to be famous.
In 331, they fought on the side of king Darius III Codomannus in his struggle against the Macedonian invader Alexander the Great; during the battle of Gaugamela (1 October), they were commanded by Phrataphernes, who surrendered his satrapy when Alexander arrived in the summer of 330. He was reappointed and is known to have been satrap of Hyrcania too in 323.
In the chaotic period after the death of Alexander (June 11, 323), Parthia was at first still ruled by Phrataphernes. When the empire was divided at Triparadisus (320), it was allotted to one Philip, but two years later, the satrap of Media, Peithon, seized the country and appointed his brother Eudamus; however, the other satraps unitedly drove them back.
From 316 on, the satrapy was ruled by Stasander, who was already satrap of Bactria, and must have seized Aria and Margiana too (which were situated between Parthia and his own satrapy). Between 308 and 303, these countries recognized the sovereignty of Seleucus I Nicator, former friend of Alexander, satrap of Babylonia and founder of the Seleucid empire. He and his descendants appointed the satraps of Parthia for more than sixty years.
In 245, a satrap named Andragoras, revolted from the young Seleucid king Seleucus II Callinicus, who had just succeeded to the throne. In the confusion, Parthia was attacked by the Parni, a nomad tribe from the Central-Asian steppe. In 238, they occupied the district known as Astavene. Three years later, a Parnian leader named Tiridates ventured further south and seized the rest of Parthia. A counter-offensive by king Seleucus in 230-227 ended in disaster, and Hyrcania was also subdued by the Parni. Their capital was Hecatompylos.
From now on, the Parni were known as Parthians. In the years that followed, their kings - Arsaces I, Arsaces II, Phriapathus, Phraates I - recognized the Seleucid king as their superiors, but under Mithradates I the Great (171-138 BCE) they conquered Media, Babylonia, and Elam. The Parthian empire was to last until 224 CE, when it was succeeded by the Sasanian empire.
An article on the history of the Parthian empire can be read here; an overview of their kings can be found here.