The battle of the Morbihan Gulf (June 56 B.C.) was the first naval battle in recorded history to definitely took place in the North Atlantic, and saw a Roman fleet raised by Julius Caesar destroy the naval power of the Veneti tribe of modern Brittany.
During the campaigning season of 57 B.C. Caesar had sent a small army under P. Crassus into western France, where he had accepted the submission of a number of tribes, amongst them the Veneti. At the end of the year Crassus had been sent into winter quarters on the Atlantic coast. During the winter he ran short of supplies and sent ambassadors out into the neighbouring states to gather supplies. The Veneti seized the two Roman ambassadors sent to them, and encouraged their neighbours to do the same. Caesar was soon faced with a rebellion of most of the maritime tribes of north-west Gaul.
The Veneti were the main naval power of Gaul. They dominated the trade with Britain, and had a fleet of solidly build ships well suited to local conditions. Caesar describes them as having shallower keels than Roman ships, which made them better suited for operations in tidal waters; as being constructed entirely of oak, with high prows and sterns which made them more able to resist the Atlantic storms, and also made them almost invulnerable to standard Mediterranean ramming tactics.
When the rebellion broke out Caesar was in Italy. He ordered Crassus to build a fleet in the Loire, and then when the weather improved moved across Gaul to join the army. A frustrating campaign followed, in which Caesar captured a number of Veneti towns, but was unable to prevent the inhabitants from being evacuated by sea, while the new Roman fleet was frequently trapped in port by summer storms, and when it did venture out apparently suffered a number of minor setbacks.
According to Caesar the Veneti ships were superior in everything but speed and the skill of their oarsmen. Not only were they almost impossible to ram, but their high sides meant that Roman missile weapons had little or no impact, while their superior seaworthiness meant that they could safely operate in waters that were too dangerous for the Romans, and could even survive being beached by the tide.
After capturing a number of towns with no effect Caesar realised that he would have to wait for the weather to improve enough for his entire fleet to join the army. Eventually the storms died down, and the Roman fleet, under Decimus Brutus, was able to sail up the coast towards Brittany. As the Roman fleet approached their remaining strongholds the Veneti decided to concentrate their entire naval force and seek a decisive battle.
Caesar doesn't actually say where the battle took place, only saying that the enemy left their harbour, and that the battle took place within sight of the main army. It is generally assumed that the battle took place in or close to Quiberon Bay, just outside the Morbihan Gulf.
After their setbacks earlier in the year the Romans had come up with a new tactic. Each ship carried its normal contingent of marines, probably taken from one of Caesar's legions, and commanded by either a tribute of the soldiers or a centurion. Just as in the First Punic War the only way the Romans were going to overcome their opponent's superior naval power was by boarding. Each Roman ship was armed with a number of sharp hooks mounted on long poles. As the two fleets closed to fighting distances these hooks were used to cut the rigging on the Veneti ships, making them vulnerable to boarding.
After a number of Veneti ships had been seized and boarded in this way the rest of the fleet attempted to make their escape, but at this crucial moment the wind failed them. This left the Veneti fleet spread out and vulnerable along the coast. The Romans took advantage of their superior speed under oars to chase down and defeat the Veneti ships one-by-one. The battle lasted from about ten in the morning until nightfall, when the surviving Veneti ships made their escape.
With their fleet destroyed the Veneti had no choice but to surrender. Caesar was not in a merciful mood – in his own account because he wanted to make sure the rights of ambassadors were respected by other tribes – but possibly because of his frustrating summer. The members of the Veneti senate were all executed and the rest of the tribe were sold into slavery.