From 55 BC, Petreius and Lucius Afranius administered the Spanish provinces as Legates, while the official governor Gnaeus Pompeius Magnus remained in Rome. After the outbreak of the Civil War in 49 BC, Petreius and Afranius marched against Caesar, who for his part wished to secure Spain before moving against Pompey in Greece. The two Legates suffered defeat after initial successes, and were forced to capitulate and disband their army on August 2 at Ilerda. Caesar allowed Petreius and Afranius their freedom, and the two travelled to Greece to join Pompey’s forces. After Pompey’s defeat in the Battle of Pharsalus, Petreius and Cato fled from the Peloponnese to North Africa, where the former continued to serve as Legate in the resistance to Caesar. Together with Titus Labienus, Petreius again achieved several successes against Caesar. After the defeat of the Pompeians at Thapsus, Petreius fled with the Numidian King, Juba. As they realized the hopelessness of their situation, Petreius and Juba resolved to take their lives on an estate near Zama: Petreius and Juba decided upon a duel, in which Petreius killed Juba. Petreius then took his own life with the help of a slave.