As a reward for his service to the triumvirs, in 54 BC, he was made one of
Julius Caesar's
legates, with whom he served for the next five years during Caesar's campaigns in
Gaul, with Caesar commenting favourably on his performance during those years. In 54 BC he accompanied Caesar during his
second expedition to
Britain, where he was placed in charge of three legions who successfully defeated a concerted attack by the forces of
Cassivellaunus. Upon Caesar's return to Gaul, Trebonius, along with one
legion, was stationed for the winter with the
Belgae at
Samarobriva. From here he accompanied Caesar in coming to the aid of
Quintus Cicero who was besieged during the beginning of
Ambiorix's revolt against Roman control of Gaul. In 53 BC, Trebonius was given a special command against the
Eburones, specifically to harass the area in the vicinity of
Huy. After the defeat of
Ambiorix, he continued to serve Caesar; in 50 BC he was placed in charge of the winter quarters in Belgic Gaul, in command of four legions while Caesar was in
Ravenna, preparing to confront Pompey and his enemies in the Senate. When Caesar heard that the
consul Gaius Claudius Marcellus had asked Pompey to defend the state against Caesar, he ordered Trebonius on 20 October 50 BC to take three legions and move to
Matisco, where he was to wait for further instructions. These arrived in April 49 BC, where Caesar instructed him to travel down to
Massilia to take command of three newly recruited legions from Northern Italy and begin the
siege of Massilia. Arriving around April 3, 49 BC, Trebonius began preparing for the siege under Caesar's eye, before the latter left Trebonius on April 14 in charge of the land assaults, while
Decimus Brutus Albinus commanded the naval forces. Before he commenced the siege, Trebonius collected labourers and cattle from the Province, ordered timber and wood suitable for wattle-work to be brought up, and then proceeded to construct the principal terrace. The siege lasted from April 19 to September 6, as Trebonius erected a
contravallation to blockade the town by land. He also constructed terraces to directly assault the walls, and used battering rams and mines to try to breach the walls. Eventually, in early July, Trebonius's men broke through the wall, and the Massiliots approached Trebonius and begged him to stop operations until the arrival of Caesar, where they would agree to capitulate to him. Trebonius, after conferring with his fellow officers, agreed to suspend the attack, as Caesar had told him under no circumstances was he to storm the town. This led to his soldiers becoming disgruntled, as they were hoping to sack the town and they blamed Trebonius for halting the attack, and they were only kept under control with great difficulty. Then in late August, Trebonius was surprised when the Massiliots burst out of the town, took advantage of the lack of guards posted around, and destroyed the siege equipment which had breached the walls. Trebonius therefore resumed the siege, and proceeded to weaken their defences. The Massiliots, hearing word of Caesar's victories in Spain, again offered to surrender, and asked Trebonius to again wait for Caesar. He agreed, although this time he insisted that the defenders should surrender their weapons, their treasure and their ships, thereby bringing the siege to an end. ==Assassin of Caesar and Liberator==