defeat and death, a force under Pacorus I assisted the Pompeian general Q. Caecilius Bassus, who was besieged at Apamea Valley by Caesarian forces. With the civil war over, Julius Caesar prepared a campaign against Parthia, but his assassination averted the war. The Parthians supported Brutus and Cassius during the ensuing Liberators' civil war and sent a contingent to fight on their side at the Battle of Philippi in 42 BC. After the Liberators' defeat, the Parthians invaded Roman territory in 40 BC in conjunction with the Roman Quintus Labienus, a former supporter of Brutus and Cassius. They swiftly overran the Roman province of Syria and advanced into Judea, overthrowing the Roman client Hyrcanus II and installing his nephew Antigonus. For a moment, the whole of the Roman East seemed lost to the Parthians or about to fall into their hands. However, the conclusion of the second Roman civil war soon revived Roman strength in Asia. Mark Antony had sent Ventidius to oppose Labienus, who had invaded Anatolia. Soon Labienus was driven back to Syria by Roman forces, and, although reinforced by the Parthians, was defeated, taken prisoner, and killed. After suffering a further defeat near the Syrian Gates, the Parthians withdrew from Syria. They returned in 38 BC but were decisively defeated by Ventidius, and Pacorus was killed. In Judaea, Antigonus was ousted with Roman help by Herod in 37 BC. With Roman control of Syria and Judaea restored, Mark Antony led a huge army into Atropatene, but his siege train and its escort were isolated and wiped out, while his Armenian allies deserted. Failing to make progress against Parthian positions, the Romans withdrew with heavy casualties. Antony was again in Armenia in 33 BC to join with the Median king against Octavian and the Parthians. Other preoccupations obliged him to withdraw, and the whole region came under Parthian control. Roman Empire vs. Parthia With tensions between the two powers threatening renewed war, Octavian and Phraataces worked out a compromise in 1 AD. According to the agreement, Parthia undertook to withdraw its forces from Armenia and to recognize a de facto Roman protectorate there. Nonetheless, Roman–Persian rivalry over control and influence in Armenia continued unabated for the next several decades. The decision of the Parthian King Artabanus III to place his son on the vacant Armenian throne triggered a war with Rome in 36 AD, which ended when Artabanus