When Was the Battle of Actium and Why Did It Happen?

"On September 2, 31 BC, at the Battle of Actium, the decisive naval battle that effectively ended the Roman Republic took place. Octavian's forces defeated those under Mark Antony and Cleopatra off the western coast of Greece." From the article:

"Octavian Attacks
An average general, Octavian entrusted his forces to his friend Marcus Vipsanius Agrippa. A skilled veteran, Agrippa began aggressively raiding the Greek coast while Octavian moved east with the army. Led by Lucius Gellius Poplicola and Gaius Sosius, Antony's fleet concentrated in the Gulf of Ambracia near Actium in what is today northwestern Greece. While the enemy was in port, Agrippa took his fleet south and attacked Messenia, disrupting Antony's supply lines. Arriving at Actium, Octavian established a position on the high ground north of the gulf. Attacks against Antony's camp to the south were easily repulsed.

A stalemate ensued for several months as the two forces watched each other. Antony's support began to wane after Agrippa defeated Sosius in a naval battle and established a blockade off Actium. Cut off from supplies, some of Antony's officers began to defect. With his position weakening and Cleopatra agitating for a return to Egypt, Antony began planning for battle. The ancient historian Dio Cassius indicates that Antony was less inclined to fight and was, in fact, seeking a way to escape with his lover. Regardless, Antony's fleet emerged from the harbor on September 2, 31 B.C.

Battle on the Water
Antony's fleet was largely composed of massive galleys known as quinqueremes. Featuring thick hulls and bronze armor, his ships were formidable but slow and hard to maneuver. Seeing Antony deploying, Octavian instructed Agrippa to lead the fleet in opposition. Unlike Antony, Agrippa's fleet consisted of smaller, more maneuverable warships made by the Liburnian people, living in what is now Croatia. These smaller galleys lacked the power to ram and sink a quinquereme but were fast enough to escape an enemy ramming attack. Moving toward each other, the battle soon began with three or four Liburnian vessels attacking each quinquereme.

As the battle raged, Agrippa began extending his left flank with the goal of turning Antony's right. Lucius Policola, leading Antony's right wing, shifted outward to meet this threat. In doing so, his formation became detached from Antony's center and opened a gap. Seeing an opportunity, Lucius Arruntius, commanding Agrippa's center, plunged in with his ships and escalated the battle. As neither side could ram, the usual means of naval attack, the fight effectively devolved into a land battle at sea. Fighting for several hours, with each side attacking and retreating, neither was able to gain a decisive advantage.

Cleopatra Flees
Watching from the far rear, Cleopatra became concerned about the course of the battle. Determining that she had seen enough, she ordered her squadron of 60 ships to put to sea. The actions of the Egyptians threw Antony's lines into disorder. Stunned at his lover's departure, Antony quickly forgot the battle and sailed after his queen with 40 ships. The departure of 100 ships doomed the Antonian fleet. While some fought on, others attempted to escape the battle. By late afternoon those that had remained surrendered to Agrippa.

At sea, Antony caught up with Cleopatra and boarded her ship. Though Antony was angry, the two reconciled and, in spite of being briefly pursued by a few of Octavian's ships, made good their escape to Egypt.

Aftermath
As with most battles from this period, precise casualties are not known. Sources indicate that Octavian lost around 2,500 men, while Antony suffered 5,000 killed and over 200 ships sunk or captured. The impact of Antony's defeat was far-reaching. At Actium, Publius Canidius, commanding the ground forces, began retreating, and the army soon surrendered. Elsewhere, Antony's allies began deserting him in the face of Octavian's growing power. With Octavian's troops closing in on Alexandria, Antony committed suicide. Learning of her lover's death, Cleopatra killed herself as well. With the elimination of his rival, Octavian became the sole ruler of Rome and was able to begin the transition from republic to empire."