Antiochus III (241-187 BC) was a Syrian king of the Seleucid dynasty, Alexander the Great's successors in Asia. Antiochus attempted to restore Alexander's empire to its former greatness, and the magnitude of this undertaking earned Antiochus the name Megas, "the Great."

The second son of Seleucus II and his wife Laodice, Antiochus was born in Babylonia. When his older brother, Seleucus III, was assassinated in Phrygia in 223, Antiochus succeeded him as king at the age of 18. His cousin Achaeus assumed military command and punished Seleucus's assassins. There was strong public support for Achaeus to ascend the throne, but he declined and remained loyal to his cousin. Achaeus governed in Asia Minor.

In 222 Molon and Alexander, the satraps of Media and Persia, revolted. Molon proclaimed himself independent and king, but by 220 his forces had abandoned him, and he and Alexander committed suicide. Then Achaeus sought power and occupied Antioch...