Caesar was, in effect, the sole power in Rome at the time of his death, in 44 B.C. In order to quell an uprising, Octavian, who at 19 was Caesar’s adopted son and sole heir, and Mark Antony, one of Caesar’s most revered army officers, joined with another close Caesar ally, Marcus Lepidus, to form the Second Triumvirate, in 43 B.C.. The three men renewed this alliance in 38 B.C., for another five-year arrangement.

The alliance did not hold, partly because of real and perceived threats. Mark Antony had married Octavian’s sister, Octavia, in order to cement the alliance between the two men. But Antony had fallen in love with the Egyptian pharaoh Cleopatra and moved to Egypt to be with her, abandoning his wife in the process. As well, Cleopatra’s son Caesarion, whose father was Julius Caesar, was very much alive and, in Octavian’s eyes, very much a possible heir to Caesar’s position of power and, therefore, a threat to the power of the Second Triumvirate in general and to Octavian in particular.

Lepidus wasn’t much of a factor in the power struggle that ensued. He gave up his legions as part of the Triumvirate arrangement that saw him named Consul and Pontifex Maximus, the title that was, in effect, high priest of the Roman religion. Octavian and Antony used those legions to defeat Brutus and Cassius, the architects of Caesar’s assassination, at the Battle of Philippi, in 42 B.C. Antony and Octavian then seized most of Lepidus’s lands. Lepidus had control of provinces and forces in Africa, but he eventually ran afoul of Octavian and was exiled.

Antony attempted to have Caesarion accepted as Caesar’s heir. Caesar had, in his will, not mentioned his son at all but had declared as his heir his great-nephew, Octavian. The two most powerful triumvirs deepened the growing chasm of distrust between them, and Antony announced in 33 B.C. that he no longer wanted to be part of the triumvirate.

The forces of Octavian and the forces of Antony were engaged in battles against separate foes several times in the two years that followed, but they eventually came to blows with each other in 31 B.C.

Antony had decamped permanently to the East, fighting against the Parthians (unsuccessfully) and the Armenians (successfully) and then consolidating his position in Egypt by marrying Cleopatra and having two children with her. Octavian, in 31, convinced the Senate to declare war against Cleopatra. That war declaration was, technically, against Cleopatra; in reality, however, the Senate had declared war against Mark Antony as well, meaning that he no longer had any legal authority in Rome.

Octavian’s main general was Agrippa, who brought with him a wealth of experience and a large number of ships and soldiers. Antony’s main ally was Cleopatra, who had a large of experience soldiers and sailors at her command.

After a few skirmishes in and around Italy and Greece, the two main forces finally met in open battle near the promontory of Actium, on the western coast of Greece. To the east was the Ambracian Gulf; to the west was the Ionian Sea. The closest large city was Nicopolis, to the north.

Sources differ on the numbers in the various armies and navies, although most sources agree that Antony had more (and larger) ships. Infantry on both sides numbered in the tens of thousands.

Agrippa had led raiding parties on nearby settlements, crippling Antony’s supply lines. Octavian had brought his land forces onto the promontory to the north of the tiny strait that connected the Ambracian Gulf with the Ionian Sea. Antony had encamped on the south side of the strait. Antony’s men built a land bridge across the narrow strip of water, and these men certainly made known their preference for a land battle. But the bulk of Octavian’s forces were on ships out in the Ionian, blocking any seaborne travel by Antony’s forces, and so a land battle would ultimately have not improved Antony’s position much. Some accounts say that Antony’s men preferred to fight a land battle anyway but that Cleopatra prevailed on Antony to press his waterborne advantage. The fact of the matter was that Antony’s forces were starving and needed to either gain new supply lines or escape to more friendly territory.

On the morning of September 2, 31 B.C., the battle began. Antony’s fleet contained huge war galleys filled with armed men. This fleet sailed out into the Ionian to confront Octavian’s fleet of smaller warships. Octavian himself was not onboard a ship but observed the battle from the shore. Antony himself was in a ship in the right wing of his fleet.

Antony’s battle plan had been to make a run for it but, if attacked, use his Octary ships to full advantage, employing the massive 16-oared ships as battering rams and artillery centers (including large ballista catapults) against Octavian’s smaller ships. But a malaria outbreak had decimated Antony’s forces, and many of those who remained were starved and otherwise hungry for home.

The smaller ships of Octavia formed enough of a bottleneck that Antony’s ships were forced to attack, and the battle raged. Octavian’s men succeeded in boarding many of Antony’s ships; but because Antony’s ships had so many soldiers onboard, the boarding of a ship did not immediately result in the seizure of a ship and so some of the fighting continued for most of the day. As well, Octavian’s smaller ships proved more maneuverable and could avoid Antony’s more serious artillery assaults.

Also in Octavian’s favor was a key defection. Quintus Dellius, one of Antony’s generals, had throw in his lot with Octavian on the eve of the battle, and so Octavian knew Antony’s battle plan. The structure of the fleet was such that Antony didn’t have many options for changing strategy.

Cleopatra herself was onboard one of the ships, and this ship, without ever engaging the enemy, made a break for it when an opportunity arose. Sensing that he was losing the battle, Antony ordered his ship to flee as well. Antony’s men did their best to flee, either in ships or over land. The resulting chaos ended in a victory for Octavian, including the burning of the remainder of Antony’s ships and the surrender of the remainder of Antony’s men.

It was the beginning of the end for Antony. He eventually ended his own life in Egypt. Octavian, on the other hand, rose to a supremacy higher than his great-uncle’s, becoming Augustus Caesar, the first emperor of Rome.