The Third Syrian War, also known as the Laodicean War, begins with one of the many succession crisis that plague the Hellenistic states.

Antiochus II leaves two ambitious mothers, his repudiated wife Laodice and Ptolemy II's daughter Berenice Syra, in a competition to put their respective sons on the throne.

Laodice claims that Antiochus had named her son heir while on his deathbed, but Berenice argues that her newly born son is the legitimate heir.

Berenice asks her brother Ptolemy III, the new Ptolemaic king, to come to Antioch and help place her son on the throne.

When Ptolemy arrives, Berenice and her child have been assassinated.

Ptolemy declares war on Laodice's newly crowned son, Seleucus II, in 246 BCE, and campaigns with great success.

In exchange for a peace in 241, Ptolemy is awarded new territories on the northern coast of Syria, including Seleucia Pieria, the port of Antioch.

The Ptolemaic kingdom is at the height of its power.