Rebellion and marriage alliances

Widespread dissatisfaction with Octavian over his soldiers' settlements encouraged many to rally at the side of Lucius Antonius, who was brother of Mark Antony and supported by a majority in the Senate.Meanwhile, Octavian asked for a divorce from Clodia Pulchra, the daughter of Fulvia and her first husband Publius Clodius Pulcher. Claiming that his marriage with Clodia had never been consummated, he returned her to her mother, Mark Antony's wife. Fulvia decided to take action. Together with Lucius Antonius she raised an army in Italy to fight for Antony's rights against Octavian. However, Lucius and Fulvia took a political and martial gamble in opposing Octavian, since the Roman army still depended on the triumvirs for their salaries. Lucius and his allies ended up in a defensive siege at Perusia (modern Perugia), where Octavian forced them into surrender in early 40 BC. Lucius and his army were spared due to his kinship with Antony, the strongman of the East, while Fulvia was exiled to Sicyon. However, Octavian showed no mercy for the mass of allies loyal to Lucius; on 15 March, the anniversary of Julius Caesar's assassination, he had 300 Roman senators and equestrians executed for allying with Lucius. Perusia was also pillaged and burned as a warning for others. This bloody event sullied Octavian's reputation and was criticized by many, such as the Augustan poet Sextus Propertius.
Sextus Pompeius, son of the First Triumvir Pompey and still a renegade general following Julius Caesar's victory over his father, was established in Sicily and Sardinia as part of an agreement reached with the Second Triumvirate in 39 BC. Both Antony and Octavian were vying for an alliance with Pompeius, who was ironically a member of the republican party, not the Caesarian faction. Octavian succeeded in a temporary alliance when in 40 BC he married Scribonia, a daughter of Lucius Scribonius Libo who was a follower of Pompeius as well as his father-in-law. Scribonia conceived Octavian's only natural child, Julia, who was born the same day that he divorced Scribonia to marry Livia Drusilla, little more than a year after his marriage.
While in Egypt, Antony had been engaged in an affair with Cleopatra and had fathered three children with her. Aware of his deteriorating relationship with Octavian, Antony left Cleopatra; he sailed to Italy in 40 BC with a large force to oppose Octavian, laying siege to Brundisium. However, this new conflict proved untenable for both Octavian and Antony. Their centurions, who had become important figures politically, refused to fight due to their Caesarian cause, while the legions under their command followed suit. Meanwhile in Sicyon, Antony's wife Fulvia died of a sudden illness while Antony was en route to meet her. Fulvia's death and the mutiny of their centurions allowed the two remaining triumvirs to effect a reconciliation. In the autumn of 40, Octavian and Antony approved the Treaty of Brundisium, by which Lepidus would remain in Africa, Antony in the East, Octavian in the West. The Italian peninsula was left open to all for the recruitment of soldiers, but in reality, this provision was useless for Antony in the East. To further cement relations of alliance with Mark Antony, Octavian gave his sister, Octavia Minor, in marriage to Antony in late 40 BC. During their marriage, Octavia gave birth to two daughters (known as Antonia Major and Antonia Minor).