Commius of the Atrebates (fl.57-50 BC) was a Gallic leader who supported Caesar for most of the Gallic War before switching sides and taking part in the final revolt under Vercingetorix. Commius first comes to our attention after Caesar's victory over the Belgic tribes on the Sambre in 57 BC. In the aftermath of the battle Caesar made Commius king of the Atrebates, having been impressed with his courage and conduct and in the belief that he would be loyal to the Romans.
Commius next appears in 55 BC when Caesar was planning his first expedition to Britain. Commius was sent across the channel with orders to visit as many states as possible and convince then to accept Roman protection. This mission ended before it began. Commius was captured almost immediately after he landed in Britain and was thrown into chains. He was only released after Caesar had successfully fought his way ashore. Commius then commanded a small force of 30 horsemen who had been part of his original entourage, using them to pursue the Britons after the failure of their attack on the Roman camp. Commius returned to Britian with Caesar in the following year. Towards the end of this second expedition he was used to negotiate the peace settlement with Cassivellaunus.
At the end of the second expedition to Britain Commius returned to Gaul. After Caesar subdued the Menapii tribe, in the Rhine delta, Commius was left in command in the area, at the head of a cavalry force. He was rewarded for his loyalty by being granted the lands of the Morini and his kingdom was made exempt from taxes.
During the winter of 53-52 BC Commius had a change of heart and joined the rebels. That winter Labienus commanded in Gaul while Caesar wintered in northern Italy, in the other half of his province. According to Caesar Labienus discovered that Commius was conspiring against Caesar and decided to attempt to trap him. Caius Volusenus Quadratus and a group of centurions were sent to meet Commius. The plan was for Volusenus to take Commius by the hand, an unusual gesture for the time. One of his centurions was to use this as an excuse to kill Commius, presumably in the hope that the death might have looked like a tragic accident and not a deliberate killing. This plan failed. Commius suffered a severe head wound, but was saved by his friends. After a tense standoff, the Gauls escaped. Commius vowed never to come within sight of a Roman.
Commius is next mentioned after the start of the siege of Alesia. When Vercingetorix called for a relief army Commius used his contacts amongst the Bellovaci to convince them to contribute 2,000 men to the army, although the rest of their army remained in the north. He was one of four men who shared the supreme command of the relief arm, a division of command that may have contributed to the Gaul's failure around Alesia.
After the fall of Alesia Commius returned to the north and joined Correus of the Bellovaci. The two men commanded the last major Gallic army to directly oppose Caesar, and for some time they managed to hold off the Romans, retreating into swamps and woods and avoiding battle. Commius travelled into Germany in an attempt to find allies, eventually returning with 500 cavalry. He survived the ambush in which Correus was killed, and when the surviving Bellovaci nobles decided to submit to Caesar he fled across the Rhine and took refuge with the same German tribe.
Commius soon returned to Gaul, at the head of a band of his surviving followers, and conducted a guerrilla campaign against the Romans, surviving on supplies captured from their convoys. The nearest Roman commander, Marcus Antonius (Mark Anthony), sent a cavalry force to catch him. After a series of minor clashes Commius suffered a serious defeat after a battle in which the Romans killed a large number of his followers after their own leader, Caius Volusenus Quadratus, was badly wounded. This convinced Commius that further resistance was useless, and he sent a message to Antonius offering to go wherever he was sent as long as he didn't have to come into the presence of any Romans. Antonius accepted these terms, and the last serious resistance to Roman rule came to an end.
At some point after this (possibly in 50 BC) Commius probably moved to Britain, where coin evidence suggests that he became king of an area south of the River Thames that included modern Hampshire and Sussex. Coin evidence also suggests that three of his sons also ruled areas of southern Britain.