The Third Syrian War was one of a series of conflicts between the Seleucid Empire and Ptolemaic Egypt, fought for control of Syria and the coast of Asia Minor. The Third Syrian War developed out of events meant to bring the two empires closer together. After the Second Syrian War, the Seleucid emperor Antiochus II had married Berenice Syra, the daughter of Ptolemy II. To do this he had had to repudiate his first wife, Laodice (thus the Laodicean War). Laodice and their children were sent to Ephesus while Berenice replaced her at court, and produced a son. The split between Laodice and Antiochus was clearly not total – in 247 BC Antiochus died while visiting her at Ephesus, while her brother Alexander was retained as general of Lydia. In the aftermath of Antichus’s death, Laodice claimed that he had named her son, Seleucus II, as his heir.
Soon after this, Ptolemy II died and was succeeded by his son Ptolemy III, Berenice’s brother. While Laodice was strong in Asia Minor, Berenice appears to have had support in Antioch. An Egyptian force from nearby Cyprus landed at Seleuceia in Pieria, the port of Antioch-on-the-Orontes, then the Seleucid capital city, and an advance force reached Antioch, but was unable to prevent the murder of Berenice and her son by supporters of Laodice.
These apparent conquests were short lived. In 245 BC Ptolemy III had to return to Egypt to deal with a rebellion in the Nile Delta. News of the death of Berenice finally leaked out, and opposition to Ptolemy grew. Seleucus II is said to have crossed the River Taurus in 244 BC, and soon regained the eastern part of what was now his empire. Antioch and Damascus were soon recovered, but Seleuceia in Pieria remained in Egyptian hands at the end of the war.
Egypt was more successful in Asia Minor, where she could use her fleet to good effect. Opposition to her there was led by Seleucus’s half brother Antiochus Hierax (the Hawk), who had been made regent of the Seleucid possessions in Asia Minor when Seleucus returned to Syria. He does not appear to have been particularly effective against the Egyptians, who ended the war in possession of southern Ionia and probably of Caria and Lycia. The exact scale of Egyptian conquests during the Third Syrian War is obscured by our lack of detailed information about the Second Syrian War, making it hard to be sure which places were conquered and which were simply retained.
The war ended in 241. Seleucus probably initiated the peace in order to deal with his many other problems. Antiochus Heirax had set himself up as an independent ruler, and was now claiming the entire empire (the resulting civil war is known as the War of the Brothers). In the east the Bactrian-Sogdian satrapy was in the process of leaving the empire under the rule of the Macedonian satraps, who felt they were better able to cope with nomad invasions if they were independent of a distant power. The vast Seleucid empire was about to shake itself apart.