Almost nothing is known of the youth…

Almost nothing is known of the youth of Ptolemy III Euergetes before 245, when, following a long engagement, he marries Berenice II, the daughter of Magas of Cyrene, thereby reuniting Egypt and Cyrenaica, which since 258 had been divided.

Shortly after his marriage and subsequent accession to the Egyptian throne, Ptolemy, avenging the murder of his sister Berenice, at once invades the Seleucid realm and annexes the eastern provinces, while his fleets sweep the coasts of Asia Minor.

A secondary purpose in his launching of the Third Syrian War, or Laodicean War, around 245, against Laodice and her son, Seleucus II, is to enforce earlier diplomatic arrangements disadvantageous to Seleucus.