Hellenistic period
Seleucid Empire

Following division of Alexander 's empire, Seleucus I Nicator received Babylonia. From there, he created a new empire which expanded to include much of Alexander's near eastern territories. At the height of its power, it included central Anatolia, the Levant, Mesopotamia, Persia, today's Turkmenistan, Pamir, and parts of Pakistan. It included a diverse population estimated at fifty to sixty million people. Under Antiochus I (c. 324/323 – 261 BC), however, the unwieldy empire was already beginning to shed territories. Pergamum broke away under Eumenes I who defeated a Seleucid army sent against him. The kingdoms of Cappadocia, Bithynia and Pontus were all practically independent by this time as well. Like the Ptolemies, Antiochus I established a dynastic religious cult, deifying his father Seleucus I. Seleucus, officially said to be descended from Apollo, had his own priests and monthly sacrifices. The erosion of the empire continued under Seleucus II, who was forced to fight a civil war (239–236 BC) against his brother Antiochus Hierax and was unable to keep Bactria, Sogdiana and Parthia from breaking away. Hierax carved off most of Seleucid Anatolia for himself, but was defeated, along with his Galatian allies, by Attalus I of Pergamon who now also claimed kingship.

The vast Seleucid Empire was, like Egypt, mostly dominated by a Greco-Macedonian political elite. The Greek population of the cities who formed the dominant elite were reinforced by emigration from Greece. These cities included newly founded colonies such as Antioch, the other cities of the Syrian tetrapolis, Seleucia (north of Babylon) and Dura-Europos on the Euphrates. These cities retained traditional Greek city state institutions such as assemblies, councils and elected magistrates, but this was a facade for they were always controlled by the royal Seleucid officials. Apart from these cities, there were also a large number of Seleucid garrisons (choria), military colonies (katoikiai) and Greek villages (komai) which the Seleucids planted throughout the empire to cement their rule. This 'Greco-Macedonian' population (which also included the sons of settlers who had married local women) could make up a phalanx of 35,000 men (out of a total Seleucid army of 80,000) during the reign of Antiochus III. The rest of the army was made up of native troops. Antiochus III ("the Great") conducted several vigorous campaigns to retake all the lost provinces of the empire since the death of Seleucus I. After being defeated by Ptolemy IV 's forces at Raphia (217 BC), Antiochus III led a long campaign to the east to subdue the far eastern breakaway provinces (212–205 BC) including Bactria, Parthia, Ariana, Sogdiana, Gedrosia and Drangiana. He was successful, bringing back most of these provinces into at least nominal vassalage and receiving tribute from their rulers. After the death of Ptolemy IV (204 BC), Antiochus took advantage of the weakness of Egypt to conquer Coele-Syria in the fifth Syrian war (202–195 BC). He then began expanding his influence into Pergamene territory in Asia and crossed into Europe, fortifying Lysimachia on the Hellespont, but his expansion into Anatolia and Greece was abruptly halted after a decisive defeat at the Battle of Magnesia (190 BC). In the Treaty of Apamea which ended the war, Antiochus lost all of his territories in Anatolia west of the Taurus and was forced to pay a large indemnity of 15,000 talents.

Much of the eastern part of the empire was then conquered by the Parthians under Mithridates I of Parthia in the mid-2nd century BC, yet the Seleucid kings continued to rule a rump state from Syria until the invasion by the Armenian king Tigranes the Great and their ultimate overthrow by the Roman general Pompey.
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