Antiochus III the Great, byname Antiochus the Great, Greek Antiochus Megas, (born 242 BCE—died 187, near Susa, Iran), Seleucid king of the Hellenistic Syrian Empire from 223 BCE to 187, who rebuilt the empire in the East but failed in his attempt to challenge Roman ascendancy in Europe and Asia Minor. He reformed the empire administratively by reducing the provinces in size, established a ruler cult (with himself and his consort Laodice as divine), and improved relations with neighbouring countries by giving his daughters in marriage to their princes.

The son of Seleucus II, Antiochus succeeded his brother Seleucus III as king. He retained from the previous administration Hermias as chief minister, Achaeus as governor of Asia Minor, and Molon and his brother Alexander as governors of the eastern provinces, Media and Persis. In the following year, when Molon rebelled and assumed the title of king, Antiochus abandoned a campaign against Egypt for the conquest of southern Syria, on the advice of Hermias, and marched against Molon, defeating him in 220 BCE on the far bank of the Tigris and also conquering Atropatene, the northwestern part of Media. Shortly thereafter he had Hermias killed and was thus rid of most of the influences from the previous administration. In the same year, Achaeus set himself up as king in Asia Minor, but a mutiny in his army kept him from attacking Antiochus.

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Antiochus was now free to conduct what has been called the Fourth Syrian War (219–216), during which he gained control of the important eastern Mediterranean sea ports of Seleucia-in-Pieria, Tyre, and Ptolemais. In 218 he held Coele Syria (Lebanon), Palestine, and Phoenicia. In 217 he engaged an army (numbering 75,000) of Ptolemy IV Philopator, a pharaoh of the Hellenistic dynasty ruling Egypt, at Raphia, the southernmost city in Syria. His own troops numbered 68,000. Though he succeeded in routing the left wing of the Egyptian army, his phalanx (heavily armed infantry in close ranks) in the centre was defeated by a newly formed Egyptian phalanx. In the subsequent peace settlement, Antiochus gave up all his conquests except the city of Seleucia-in-Pieria.

After the Syrian war, he proceeded against the rebel Achaeus. In alliance with Attalus I of Pergamum, Antiochus captured Achaeus in 213 in his capital, Sardis, and had him executed in a barbaric manner. After the pacification of Asia Minor he entered upon his later to be famous eastward campaign (212–205), pressing forward as far as India. In 212 he gave his sister Antiochis in marriage to King Xerxes of Armenia, who acknowledged his suzerainty and paid him tribute. He occupied Hecatompylos (southeast of the Caspian Sea), the capital of the Parthian king Arsaces III, and forced him to enter into an alliance in 209 and the following year defeated Euthydemus of Bactria, though he allowed him to continue to rule and retain his royal title. In 206 he marched across the Hindu Kush into the Kābul Valley and renewed a friendship with the Indian king Sophagasenos.

Returning westward via the Iranian provinces of Arachosia, Drangiana, and Carmania, he arrived in Persis in 205 and received tribute of 500 talents of silver from the citizens of Gerrha, a mercantile state on the east coast of the Persian Gulf. Having established a magnificent system of vassal states in the East, Antiochus now adopted the ancient Achaemenid title of “great king,” and the Greeks, comparing him to Alexander the Great, surnamed him also “the Great.”

After the death of Ptolemy IV, Antiochus concluded a secret treaty with Philip V, ruler of the Hellenistic kingdom of Macedonia, in which the two plotted the division of the Ptolemaic empire outside Egypt. Antiochus’ share was to be southern Syria, Lycia, Cilicia, and Cyprus; Philip was to have western Asia Minor and the Cyclades. Antiochus invaded Coele Syria, defeated the Ptolemaic general Scopas at Panion near the source of the Jordan River in the year 200, gained control of Palestine, and granted special rights to the Jewish temple state. But Philip, marching along the Dardanelles, became involved in a war with Rhodes and Pergamum, both of whom appealed to Rome for help against Macedonia, informing Rome of the alliance between the two Hellenistic kings. Rome intervened decisively in the system of Hellenistic states. Philip was defeated by the Romans in the Second Macedonian War (200–196), and Antiochus refused to help him. Instead, taking advantage of the Romans’ involvement with Philip, Antiochus marched against Egypt. Though the Romans had sent ambassadors to Ptolemy V, they could not lend him any serious assistance. When peace was concluded in 195, Antiochus came permanently into possession of southern Syria—which had been fought over for 100 years by the Ptolemies and Seleucids—and of the Egyptian territories in Asia Minor. He also gave his daughter Cleopatra in marriage to Ptolemy V. Egypt practically became a Seleucid protectorate.

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In his insatiable expansionist drive, Antiochus occupied parts of the kingdom of Pergamum in 198 and in 197 Greek cities in Asia Minor. In 196 BCE he crossed the Hellespont into Thrace, where he claimed sovereignty over territory that had been won by Seleucus I in the year 281 BCE. A war of harassment and diplomacy with Rome ensued. A number of times the Romans sent ambassadors demanding that Antiochus stay out of Europe and set free all the autonomous communities in Asia Minor. To meet these demands would have meant the actual dissolution of the western part of the Seleucid Empire, and Antiochus thus refused. Tensions with Rome increased further when the great Carthaginian general Hannibal, who had fled from Carthage in the aftermath of defeat by the Romans in the Second Punic War, found refuge with Antiochus in 195 BCE and became his adviser.

Antiochus offered an alliance to Philip of Macedonia, whom he had previously forsaken, but was rebuffed. Philip, Rhodes, Pergamum, and the Achaean League joined Rome. Only the Aetolians, discontent with Rome’s growing influence in Greece, called upon Antiochus to be their liberator and appointed him commander in chief of their league. Relying on them Antiochus landed in Demetrias in the autumn of 192 with only 10,500 men and occupied Euboea. But he found little support in central Greece. In 191 the Romans, numbering more than 20,000, cut him off from his reinforcements in Thrace and outflanked his position at the pass of Thermopylae (in Greece). With the remainder of his troops Antiochus fled to Chalcis on Euboea and from there by sea to Ephesus; his fleet was wiped out by the combined naval forces of Rome, Rhodes, and Pergamum. Meeting no resistance, the Roman army crossed the Hellespont in 190. Antiochus was now eager to negotiate on the basis of Rome’s previous demands, but the Romans insisted that he first evacuate the region west of the Taurus Mountains. When Antiochus refused, he was decisively defeated in the Battle of Magnesia near Mt. Sipylus, where he fought with a heterogeneous army of 70,000 men against an army of 30,000 Romans and their allies. Although he could have continued the war in the eastern provinces, he renounced all claim to his conquests in Europe and in Asia Minor west of the Taurus at the peace treaty of Apamea. He also was obliged to pay an indemnity of 15,000 talents over a period of 12 years, surrender his elephants and his fleet, and furnish hostages, including his son Antiochus IV. His kingdom was now reduced to Syria, Mesopotamia, and western Iran. In 187 Antiochus was murdered in a Baal temple near Susa, where he was exacting tribute in order to obtain much needed revenue.