Ptolemy II was the son of Berenice and Ptolemy I (Soter), who had been a trusted commander under Alexander the Great and who had wrested Egypt from Antigonus upon the death of Alexander in 323 B.C., proclaiming himself king. Ptolemy II Philadelphus, which means 'Brother/Sister-loving', was the second ruler of the Ptolemaic Dynasty. He was made co-ruler with his father in 285 B.C. and succeeded to the throne in 283 B.C. Ptolemy's first wife, Arsinoö I, daughter of Lysimachus, was the mother of his legitimate children.
One of his first engagements was a war with his half-brother Magas, who had governed Cyrene as viceroy under Ptolemy Soter, and had attempted to invade Egypt on the death of the latter. Magas was supported by Antiochus II., king of Syria and the war was at length terminated by a treaty, which left Magus in undisputed possession of the Cyrenaica, while his infant daughter Berenice was betrothed to Ptolemy, the son of Philadelphus.
Philadelphus concluded a treaty with the Romans and engaged in hostilities with Syria, which were terminated towards the close of his reign by a peace treaty, by which Ptolemy gave his daughter Berenice in marriage to Antiochus II. Egypt was left the dominant naval power of the eastern Mediterranean; the Ptolemaic sphere of power extended over the Cyclades to Samothrace, and the harbours and coast towns of Cilicia Trachea ("Rough Cilicia"), Pamphylia, Lycia and Caria.
Ptolemy increased size and prestige of the famed library of Alexandria and patronized scientific research and literature. He had strange beasts from far off lands sent to his palace to add to his collection. He surrounded himself with poets, philosophers and scientists, including the mathematician Euclid. Tradition connects Ptolemy with the Septuagint translation of the Old Testament into Greek. According to the "Letter of Aristeas", Ptolemy II requested 70 Jewish scholars come from Jerusalem to translate the Pentateuch into a Greek version to be placed into the Great Library collection.
Several new cities or colonies were founded by Philadelphus. On the Red Sea there were least two bearing the name of Arsinoö, one called after another of his sisters Philotera, and two cities named in honour of his mother Berenice. The same names occur also in Cilicia and Syria, where he built the important fortress of Ptolemais in Palestine.
A the close of his reign Philadelphus had a standing army of 200,000 foot, and 40,000 horse, besides war-chariots and elephants; a fleet of 1500 ships; and a sum of 740,000 talents in his treasury.
Probably for political reasons, he banished his first wife Arsinoö, the daughter of Lysimachus, to Coptos in Upper Egypt on a charge of conspiracy. After her removal Ptolemy married his own sister Arsinoö, the widow of Lysimachus. The practise of marrying one's own sister was an Egyptian custom which was abhorrent to the Greeks. Never the less, it was a custom that was continued until the end of the dynasty in 30 B.C.
Ptolemy Philadelphus died on January 29, 246 BC.