Battle of Actium - Wikipedia

On September 2, 31 BC, 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:

"Antony meant to anticipate an attack by a descent upon Italy towards the end of 32 BC, and went as far as Corcyra. However, finding the sea guarded by a squadron of Octavian's ships, he retired to winter at Patrae while his fleet for the most part lay in the Ambracian Gulf and his land forces encamped near the promontory of Actium, while the opposite side of the narrow strait into the Ambracian Gulf was protected by a tower and troops.[8]

After Octavian's proposals for a conference with Antony had been scornfully rejected, both sides prepared for the struggle the next year. The early months passed without notable event, beyond some successes of Agrippa on the coasts of Greece, meant to divert Antony's attention. It was not until the latter part of August that troops were landed in the neighborhood of Antony's camp on the north side of the strait. Still, Antony could not be tempted out. It took some months for his full strength to arrive from the various places in which his allies or his ships had wintered, and during these months not only was Agrippa continuing his descent upon Greek towns and coasts but in various cavalry skirmishes Octavian had so far prevailed, so that Antony abandoned the north side of the strait and confined his soldiers to the southern camp. Cleopatra now earnestly advised that garrisons should be put into strong towns and that the main fleet should return to Alexandria. The large contingent furnished by Egypt gave her advice as much weight as her personal influence over Antony, and it appears that this movement was agreed to.[8]

Octavian learned of this and debated how to prevent it. At first of a mind to let Antony sail and then attack him, he was prevailed upon by Agrippa to give battle.[9] On 1 September he issued an address to his fleet, preparing them for battle. The next day was wet and the sea was rough. When the trumpet signal for the start rang out, Antony's fleet began issuing from the straits and the ships moved into line and remained quiet. Octavian, after a short hesitation, ordered his vessels to steer to the right and pass the enemy's ships. For fear of being surrounded, Antony was forced to give the word to attack."