After the defeat of the Republicans at Philippi, and Sextus Pompey in Sicily, Octavian set about organizing the west under his control. In the meantime, Marcus Antonius moved east to do the same, and to seek further glory against Parthia. Antony had met with Cleopatra of Egypt as early as 41 BC in Tarsus, and while Octavian struggled with his own problems, including Antony's brother, in Italy, Antony was seemingly satisfied to allow the east to rest in political turmoil. He spent the winter of 41 and 40 BC with Cleopatra in Egypt, where she bore him twin children, and their affair blossomed.
Before the victory at Philippi, however, the Republican general Cassius had sent Q. Labienus, the son of Caesar's Legate Titus Labienus, into Parthia to solicit support for their cause. With the unstable political climate in both the west and east, and Antony mostly idle in Egypt, Orodes the King of Parthia, commissioned a joint command under his son Pacorus and Labienus to invade Roman territory. In early 40 BC, the Parthians took Syria, while Labienus continued south into Judaea and eventually west all the way to Asia Minor. At this point, the situation with Sextus Pompey in Sicily had come to a head, and Antony was forced to act. Ignoring Labienus and the Parthians for the time being, Antony moved west to aid Octavian. While Antony was in the west however, where they would eventually be victorious in 36 BC, he sent a command under P. Ventidius Bassus to deal with Labienus.
Bassus proved an effective commander and dealt with Labienus swiftly and effectively. In three consecutive battles in 39 BC, Bassus defeated this final remnant of Republican resistance in Asia Minor. Over the next two years, Bassus reclaimed lost territory, pushing his way into Syria and forcing the Parthians to withdraw. In 38 BC, Bassus met Pacorus at the Battle of Gindarus in Mesopotamia, ironically on June 9, the anniversary of Crassus' great defeat at Carrhae. This time the Romans won a resounding victory, in which Pacorus was killed, and by the time Antony returned for good in 36 BC, the east was essentially secured. With the death pf Pacorus, King Orodes of Parthia abdicated his throne in favor of a second son Phraates, but Phraates had his father murdered to ensure his rule, and Antony felt the time was right for revenge and invasion.
The following campaign year, 37 BC, he initiated a campaign similar to his eastern predecessor, Pompey the Great. The tribes of the Caucasus region were subdued in order to secure peace from the north, and rather than follow Crassus' disastrous course into Mesopotamia, Antony marched east through Armenia. By 36 BC, he had secured the support of Artavastes, King of Armenia and the two continued the campaign into the Parthian territory of Media. Antony's main force moved ahead of the baggage train, which was left in the protection of Artavastes, and while Antony besieged Phraaspes, the Parthian capital of Media, the Parthians wisely turned on Artavastes. The vital supply train was captured, and Artavastes retreated to Armenia, leaving Antony to fend for himself in the rough and hostile Parthian territory. Antony knew his invasion had failed and was forced to withdraw, losing nearly one quarter of his men in the process. Unlike Crassus, however, Antonius maintained his command integrity and extricated the survivors mostly intact, and without major incident.
In the meantime, Octavian was having his own success in a campaign against Illyrian tribes, and his prestige continued to grow. On the political stage, the situation with Cleopatra was beginning to cast Antony as a sell-out against Roman culture, and Octavian would seize at the opportunity to publicly chastise him. While still married to Octavia, with whom Antony had 2 daughters; Cleopatra bore him another son in 36 BC. The following year, Octavia brought reinforcements from her brother to aid Antony, but he sent her back to Italy in an obvious slight against Octavian, and the shaky peace was beginning to unravel. Antony had seemingly begun to take to eastern culture on a grand scale, adorning himself in Egyptian dress, practicing eastern customs while ignoring those of Rome, and doling out great gifts to Cleopatra. Among these gifts was the division of the eastern empire under Cleopatra's children. Each child, Caesarion the son of Caesar included, was placed as regent of various kingdoms effectively making Cleopatra the Queen mother of several Roman territories. By 34 BC, Antony seized Armenia from his former ally Artavastes and staged a magnificent spectacle in Alexandria to commemorate the event.
In these "Donations of Alexandria" Antony declared Cleopatra as the Queen of Kings, and Caesarion as King of Kings. In so doing, he sought to undermine Octavian's claim as Caesar's heir by recognizing Caesarion as Caesar's legitimate son. This was seemingly the final straw against Octavian, and the propaganda campaign in Rome against Antony's outrages began in earnest. Antony was portrayed as a pawn of Cleopatra, the foreign Queen who, it was said, sought to become Queen of Rome. Antony had declared himself the human incarnation of Dionysus (Bacchus in Roman mythos), and Cleopatra claimed to be Aphrodite (Venus). This had relevance in Egyptian culture as the two Greek gods were often associated with the gods of nobility Osiris and Isis.
While initially, these declarations seem to have little impact on Antony's supporters in the east, they did allow Octavian to vilify him in the west. The gifting of client kingdoms to important foreign magistrates was certainly nothing new in Roman culture, but placing so much territory within the influence of one royal family certainly was cause for consternation. While Octavian was busy solidifying his western popularity through great building and maintenance projects conducted by Agrippa, Antony continued to slip in the views of the traditional Roman. In what may have been the final proof needed, Antony seemingly married Cleopatra, at least by 32 BC, even before officially divorcing (the mistreated virtuous Roman wife) Octavia in the same year. Unlike Caesar before him, who propped Cleopatra up politically, but refused to acknowledge her in any official capacity under Roman law, (as it was illegal for a Roman to marry a foreigner), Antony plunged head first into direct enmity with Octavian. Though Octavian would use political guile to gain support for his cause, the door to the final civil war of the Roman Republic was wide open.