Octavian's power was further enhanced after his victory against the combined fleets of Mark Antony and Cleopatra, Queen of Egypt, in the Battle of Actium in 31 BC, where Antony had assembled 500 ships against Octavian's 400 ships. This last naval battle of the Roman Republic definitively established Octavian as the sole ruler over Rome and the Mediterranean world. In the aftermath of his victory, he formalized the Fleet's structure, establishing several key harbors in the Mediterranean (see below). The now fully professional navy had its main duties consist of protecting against piracy, escorting troops and patrolling the river frontiers of Europe. It remained however engaged in active warfare in the periphery of the Empire.