Cleopatra and her younger brother Ptolemy XIII acceded to the throne as joint rulers, but a fallout between them led to open civil war. Cleopatra fled briefly to Roman Syria in 48 BC but returned later that year with an army to confront Ptolemy XIII. As a Roman client state, Roman statesman Pompey the Great planned Ptolemaic Egypt as a place of refuge after losing the 48 BC Battle of Pharsalus in Greece against his rival Julius Caesar in Caesar's Civil War. However, Ptolemy XIII had Pompey killed at Pelousion and sent his severed head to Caesar, while the latter occupied Alexandria in pursuit of Pompey. With his authority as consul of the Roman Republic, Caesar attempted to reconcile Ptolemy XIII with Cleopatra. However, Ptolemy XIII's chief adviser Potheinos viewed Caesar's terms as favoring Cleopatra. So his forces, led first by Achillas and then Ganymedes under Arsinoe IV (Cleopatra's younger sister), besieged both Caesar and Cleopatra at the palace. Reinforcements lifted the siege in early 47 BC and Ptolemy XIII died shortly afterwards in the Battle of the Nile. Arsinoe IV was eventually exiled to Ephesus and Caesar, now an elected dictator, declared Cleopatra and her younger brother Ptolemy XIV as joint rulers of Egypt. However, Caesar maintained a private affair with Cleopatra that produced a son, Caesarion (later Ptolemy XV), before he departed Alexandria for Rome.