The battle of Lutetia (May 52 B.C.) was a victory won by Labienus, Caesar's most able lieutenant during the Gallic War, over the Senones and Parisii on the left bank of the Seine close to the centre of modern Paris.
After the capture of Avaricum Caesar split his army in half. He took six legions south, into the lands of the Arverni, to besiege Gergovia on the Allier, while Labienus was sent north, with four legions, into the lands of the Senones and Parisii.
Labienus decided to set up his main base at Agendicum (now Sens, on the Yonne River fifty miles to the south east of Paris). Leaving his baggage and his newest recruits at Agendicum he then advanced towards the Parisii town of Lutetia, built on the Ile de la Cité in the heart of modern Paris. Labienus's line of advance took him along the left bank of the Seine.
As the Romans advanced towards the town a strong Gallic army was formed to oppose them. Command of this army was given to the elderly but experienced Senones chief Camalugenus. Lutetia was protected by marshes, which Camalugenus used to stop the Roman advance.
After a futile attempt to force his way across the marsh, Labienus decided to attempt to outflank the Gauls. He pulled back up the Seine to Melodunum (modern Melun, twenty miles south-east of Lutetia). Most of the fighting men of Melun were away with the army at Lutetia, and so the Romans were able to capture the town without much difficulty and begin their march up the right bank of the Seine back towards Lutetia.
Although Labienus had successfully crossed the river, he was still unable to capture Lutetia. As the Romans approached the town Camalugenus ordered it to be burnt down. The bridges across the Seine were destroyed, and the Gauls took up a new position on the south bank of the river opposite the Roman camp.
Events further south now intervened and placed the entire Roman expedition in the north in jeopardy. The Aedui tribe, Rome's firmest ally in Gaul, joined the revolt. Caesar was forced to abandon the siege of Gergovia, and was believed to be trapped on the wrong side of the Loire to help Labienus.
This news encouraged the Bellovaci tribe to join the revolt. Labienus was now trapped between two Gallic armies – the Bellovaci to his north and the Senones and Parisii to his south. He was forced to abandon any plans for further campaigns in the north and instead find a way to fight his way back across the Seine and back to his baggage at Agendicum.
Labienus decided to try and convince the Gauls that he was planning to march up the Seine, while his main force actually crossed the river downstream from Lutetia. Overnight the boats he had used to cross the river originally were quietly floated down the river. Five cohorts of infantry and a flotilla of small boats were then ordered to move noisily upstream. Five more cohorts guarded the camp while the remaining three legions marched quietly downstream to the boats. Catching the Gallic scouts by surprise all three legions and their cavalry were quickly shipped across the river and were able to form up ready to defend themselves against the inevitable Gallic attack.
Camalugenus responded by splitting his own force. One part was sent upstream to watch the Roman diversion and one remained in the Gallic camp, but the largest part of the army advanced downstream to attack the Romans. Caesar mentions two of the three legions involved. The Seventh Legion was posted on the Roman right, where it won a quick victory, forcing its opponents to turn and flee. The Twelfth Legion, on the Roman left (presumably closest to the river), was harder pressed by a force led by Camalugenus.
The battle was decided by the Seventh Legion, which having won its own battle turned to its left and attacked Camalugenus from the rear. The Gallic right was surrounded, but refused to surrender and was virtually wiped out. Camalugenus was amongst the dead.
When the force that was guarding the Gallic camp discovered that a battle had broken out they advanced towards the battlefield, but they arrived too late to make any difference to the fighting. Instead they attempted to make a stand on a hill behind the original Gallic lines, but were swept away by the victorious Roman legions.
Having defeated the Senones and Parisii Labienus retreated back to his camp at Agendicum, where he collected his baggage and the new recruits. His force then moved to join Caesar, and the reunited Roman army moved south-east to the decisive siege of Alesia.