In the years before the battle, Rome was ruled by the so-called Second Triumvirate of Octavian, Marc Anthony and Lepidus.
This was an uneasy alliance and broke down when Marc Anthony fell in love with queen Cleopatra VII of Egypt and soon after made efforts to establish an eastern Roman empire, independent of Rome.
Octavian deposed Lipedus, waged a propaganda war against Marc Anthony and then both sides prepared for war.
In 31 BCE two vast armies (close to 20 legions each) and sizeable navies converged on each other in and near Epirus, outside the Ambracian Gulf.
Marc Anthony tried to do battle, but Octavian held off and the result was a stalemate that lasted several months.
Octavians admiral Agrippa defeated his opponent Sosius off the western Peleponnesos and set up a naval blockade, cutting off Marc Anthony's line of communications.
The new Egyptian great king started to run out of food for his large army and had to break out.
The result was the naval battle of Actium.
Marc Anthony's fleet, led by admiral Antonius, numbered 500 ships, though only 230 of them were war galleys.
These were large quinqueremes and quadriremes.
However malaria had thinned the ranks of his oarsmen before the battle.
Marc Anthony had burned some of these ships and was probably left with only 140, still not fully manned.
This meant that they could not go full speed and employ ramming tactics to full extent.
Still, they were formidable battle platforms, bronze-armored and nearly impossible to ram, armed with several ballistae.
They were big and gave a height advantage for their marines and archers.
Octavian's force, led by Agrippa, had around 260 warships.
Most of these were smaller liburnae, about half the size of the quinqueremes.
They were less powerful, but more maneuverable.
To take advantage of the northwestern wind, Antonius had no choice but to move out of the protection of the gulf.
He had the sails of his ships lowered and deployed in two lines, war galleys up front and transports plus Cleopatra's squadron in the rear.
Agrippa also formed two lines, in a crescent formation to contain the enemy.
Antonius had been planning to break through at the right flank, using his biggest ships in the attack.
But one of Marc Anthony's generals had defected to Octavian and briefed him of the plan, so Agrippa's fleet stayed out of range.
After midday, Antonius could wait no longer and advanced his left wing.
Agrippa responded by skirmishing with the enemy and luring them further into open water, where the increased space gave an advantage to his greater numbers and maneuverability.
Both sides thinned and extended their lines in order not to be outflanked.
The battle raged for hours, indecisively.
With ramming made impossible for both sides, it was a mix of artillery fire and boarding fights.
In all the maneuvering a gap opened up in the center.
Cleopatra could not bear the stress and hurriedly fled through it with her squadron, followed by the supply ships.
Marc Anthony himself boarded a quinquereme and escaped too.
It is unclear whether the two lovers seized an opportunity, or had planned an escape beforehand.
Anyway the double flight disheartened his fleet, which started to crumble.
Octavian's liburnae shot fire arrows at the enemy ships and took them out one by one.
He spent the whole night helping to save lives of marines aboard the burning vessels.
On the next day, men of Marc Anthony's accompanying land army, deprived of naval support, had either fled or were captured.
It was just one battle lost, but Marc Anthony had forfeited valuable territory and prestige.
He could not rally his full army, was hunted down an beaten in Egypt a year later.
Cleopatra was captured but committed suicide by eating poisoned dates.
His rivals eliminated, Octavian became Augustus Caesar, emperor of Rome.
War Matrix - Battle of Actium
Roman Ascent 200 BCE - 120 CE, Battles and sieges