Gaius Octavius Augustus, a grand-nephew and adoptive son of Gaius Julius Caesar, gained the reign over the western part of the Roman Empire within the purview of the second triumvirate in October of 40 BC. Lepidus gained the reign over Africa and Marcus Antonius over the eastern part of the Roman Empire. This triumvirate was renewed for another five years in 37 BC. But ... Octavian seized Sicily and Africa the following year and so Marcus Antonius was the last obstacle for him in his way to absolute power.
The wedding of Marcus Antonius and Cleopatra caused a war
Marcus Antonius married Cleopatra according to Egyptian rituals in October 37 BC. But he was already married with Octavian’s sister Octavia the Younger in Rome. He married her after the death of his first wife Fulvia. So Cleopatra concentrated great power into her hands.
Octavian wasn’t going to tolerate that and he regarded the Egyptian squandering money and the marriage of Antonius and Cleopatra as a personal offence and a call to fight. Antonius tried to avoid an armed conflict and sent a report of the administration of Empire’s eastern part to Roman Senate at the end of 33 BC. But Octavian put this report to a tough critique.
As well as Caesar before, Marcus Antonius wanted to divide the eastern Mediterranean into several provinces subordinate directly to Rome. These provinces shouldn’t be so large, but their territory should be strongly controlled and their border should be easy to defend. The provinces would have been protected by a zone of allied kingdoms, which would have been ruled by domestic dynasties with a direct support and supervision of Rome.
Antonius and Cleopatra concentrated large naval and ground forces in Ephesus on the western coast of Asia Minor in spring of 32 BC. But two Antonius’ fellow fighters (Plancus and Titius) defected to the enemy at the beginning of the summer. They told Octavian that Antonius made a will, in which he recognized Caesarion as a son of Caesar, left a great property to Cleopatra’s children and demanded to be buried alongside Cleopatra in Alexandria. Octavian skilfully presented this last part of the will to the Romans, who were very outraged. Octavian mobilized the Roman armies in October 32 BC and declared the war on Cleopatra.
Marcus Antonius and Octavian Augustus