Gauls
In 58 BC Julius Caesar launched the Gallic Wars and had conquered the whole of Gaul by 51 BC. He noted that the Gauls (Celtae) were one of the three primary peoples in the area, along with the Aquitanians and the Belgae. Caesar's motivation for the invasion seems to have been his need for gold to pay off his debts and for a successful military expedition to boost his political career. The people of Gaul could provide him with both. So much gold was looted from Gaul that after the war the price of gold fell by as much as 20%. While they were militarily just as brave as the Romans, the internal division between the Gallic tribes guaranteed an easy victory for Caesar, and Vercingetorix's attempt to unite the Gauls against Roman invasion came too late. After the annexation of Gaul a mixed Gallo-Roman culture began to emerge.
Roman navy
In 56 BC, for the first time a Roman fleet engaged in battle outside the Mediterranean. This occurred during Julius Caesar's Gallic Wars, when the maritime tribe of the Veneti rebelled against Rome. Against the Veneti, the Romans were at a disadvantage, since they did not know the coast, and were inexperienced in fighting in the open sea with its tides and currents. Furthermore, the Veneti ships were superior to the light Roman galleys. They were built of oak and had no oars, being thus more resistant to ramming. In addition, their greater height gave them an advantage in both missile exchanges and boarding actions. In the event, when the two fleets encountered each other in Quiberon Bay, Caesar's navy, under the command of D. Brutus, resorted to the use of hooks on long poles, which cut the halyards supporting the Veneti sails. Immobile, the Veneti ships were easy prey for the legionaries who boarded them, and fleeing Veneti ships were taken when they became becalmed by a sudden lack of winds. Having thus established his control of the English Channel, in the next years Caesar used this newly built fleet to carry out two invasions of Britain.
The last major campaigns of the Roman navy in the Mediterranean until the late 3rd century AD would be in the civil wars that ended the Republic. In the East, the Republican faction quickly established its control, and Rhodes, the last independent maritime power in the Aegean, was subdued by Gaius Cassius Longinus in 43 BC, after its fleet was defeated off Kos. In the West, against the triumvirs stood Sextus Pompeius, who had been given command of the Italian fleet by the Senate in 43 BC. He took control of Sicily and made it his base, blockading Italy and stopping the politically crucial supply of grain from Africa to Rome. After suffering a defeat from Sextus in 42 BC, Octavian initiated massive naval armaments, aided by his closest associate, Marcus Agrippa: ships were built at Ravenna and Ostia, the new artificial harbor of Portus Julius built at Cumae, and soldiers and rowers levied, including over 20,000 manumitted slaves. Finally, Octavian and Agrippa defeated Sextus in the Battle of Naulochus in 36 BC, putting an end to all Pompeian resistance.
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.