Berenike was the daughter of Ptolemy II Philadelphos and his first wife, Arsinoe I. Her mother’s apparent treason cast suspicion on her as a traitor’s daughter and made her legitimacy questionable. Ptolemy arranged to have Berenike and her siblings legally adopted by his second wife (and sister) Arsinoë II after her death, ensuring their high status once more. When she was still a child, Berenike’s horses won victories in the Isthmean, Nemean, and Olympic games.*
In 252 she became the second wife of Antiochos II as part of a peace agreement between him and her father. As part of this agreement, Antiochos would divorce his first wife, Laodike I and any children he had with Berenike would inherit the throne before his children with Laodike. We know little of the years of their marriage other than the fact that they did have at least one child together, a son named Antiochos.
This stability couldn’t last. Ptolemy II’s death signaled the end for Berenike and her son. Antiochos II left her to return to Laodike, only to die six months later, possibly by poison. Berenike had little to no protection against Laodike and her sons and she and her own son were soon killed, almost certainly with the involvement of both Laodike and her son Seleukos II. The peace between the Ptolemies and the Seleukids was broken as Ptolemy III invaded to avenge his sister and nephew, sparking off the Third Syrian War.
*Some have suggested that this might actually have been Berenike II.
Shipley, Graham. The Greek World after Alexander 323-30 B.C. New York: Routledge, 2000.
Pomeroy, Sarah. Goddesses, Whores, Wives, and Slaves: Women in Classical Antiquity. New York: Pantheon Books, 1995.
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Berenice Phernophorus - Livius.org
Berenice (Seleucid queen) - Wikipedia