Against the intention of the Egyptian queen, he supported in the Roman civil war Gaius Cassius Longinus, but had to take refuge in Tyre and was finally handed over to Cleopatra in 41 BC.
When Caesar sided with Cleopatra in her dispute with her brother Ptolemy XIII the minister Pothinus ordered Achillas to march with his strong army from Pelusium to Alexandria (autumn 48 BC).
Because Caesar had not enough soldiers for a military confrontation in an open battle he forced Ptolemy XIII to send two negotiators of high rank to Achillas.
[3] The ships, that Serapion and some cities, for example Tyre, had sent, enabled Cassius to beat decisively the Caesarian general Publius Cornelius Dolabella (July 43 BC).
The historian Michael Grant believes that Serapion tried to support Arsinoe IV, who was then living in exile in the temple of Artemis in Ephesus, against her older hostile sister Cleopatra and perhaps even wanted to make her new queen of Egypt.