War with Pompeius

A denarius of Sextus Pompeius, minted for his victory over Octavian's fleet. On the obverse the Pharus of Messina, who defeated Octavian. On the reverse, the monster Scylla

Sextus Pompeius threatened Octavian in Italy by denying to the peninsula shipments of grain through the Mediterranean; Pompeius' own son was put in charge as naval commander in the effort to cause widespread famine in Italy. Pompeius' control over the sea prompted him to take on the name Neptuni filius, "son of Neptune". A temporary peace agreement was reached in 39 BC with the treaty of Misenum; the blockade on Italy was lifted once Octavian granted Pompeius Sardinia, Corsica, Sicily, and the Peloponnese, and ensured him a future position as consul for 35 BC. The territorial agreement amongst the triumvirs and Sextus Pompeius began to crumble once Octavian divorced Scribonia and married Livia on 17 January 38 BC. One of Pompeius' naval commanders betrayed him and handed over Corsica and Sardinia to Octavian. However Octavian needed Antony's additional support to attack Pompeius, so an agreement was reached with the Second Triumvirate's extension for another five-year period beginning in 37 BC. In supporting Octavian, Antony expected to gain support for his own campaign against Parthia, desiring to avenge Rome's defeat at Carrhae in 53 BC. In an agreement reached at Tarentum, Antony provided 120 ships for Octavian to use against Pompeius, while Octavian was to send 20,000 legionaries to Antony for use against Parthia. However, Octavian sent only a tenth the number of those promised, which was viewed by Antony as an intentional provocation.
Octavian and Lepidus launched a joint operation against Sextus in Sicily in 36 BC. Despite setbacks for Octavian, the naval fleet of Sextus Pompeius was almost entirely destroyed on 3 September by general Agrippa at the naval battle of Naulochus. Sextus fled with his remaining forces to the east, where he was captured and executed in Miletus by one of Antony's generals the following year. Both Lepidus and Octavian gathered the surrendered troops of Pompeius, yet Lepidus felt empowered enough to claim Sicily for himself, ordering Octavian to leave. However, Lepidus' troops deserted him and defected to Octavian since they were weary of fighting and found Octavian's promises of money to be enticing. Lepidus surrendered to Octavian and was permitted to retain the office of pontifex maximus (head of the college of priests), but was ejected from the Triumvirate, his public career at an end, and was effectively exiled to a villa at Cape Circei in Italy. The Roman dominions were now divided between Octavian in the West and Antony in the East. To maintain peace and stability in his portion of the Empire, Octavian ensured Rome's citizens of their rights to property. This time he settled his discharged soldiers outside of Italy while returning 30,000 slaves to former Roman owners that had previously fled to Pompeius to join his army and navy. To ensure his own safety and that of Livia and Octavia once he returned to Rome, Octavian had the Senate grant him, his wife, and his sister tribunal immunity, or sacrosanctitas.