Since Seleucus' legitimate heir, Demetrius I Soter, was still a hostage in Rome, Antiochus, with the help of King Eumenes II of Pergamum, seized the throne for himself, proclaiming himself co-regent for another son of Seleucus, an infant named Antiochus (whom he then murdered a few years later).
The Battle of Mount Olympus was fought in 189 BC between the Galatian Gauls of Asia Minor and an alliance consisting of Rome and Pergamum.