Answer
Control of Cyrenaica, the eastern province of Ptolemaic Egypt, was lost to Ptolemy II after the defection of his half-brother Magas. This defection occurred around 275 BCE, as a response to Ptolemy II's decision to marry Arsinoe II, Magas' former wife, after the death of Lysimachus, King of Thrace, who originally married Arsinoe II.