The siege of Massilia (March-September 49 BC) was an early victory for Caesar during the Great Roman Civil War, largely won by his subordinates while Caesar himself campaigned in Spain.
Massilia (modern Marseille) was a ancient Greek city that had become a valuable Roman ally in southern Gaul. The defence of Massilia was led by Lucius Domitius Ahenobarbus, a long standing enemy of Caesar, and the Senate's chose as governor of Transalpine Gaul. Caesar had already dispersed an army commanded by Domitius during his advance on Rome. Domitius had responded by raising a small force of his own supporters, and setting sail for Massilia in seven galleys.
According to Caesar's own account of events he was already in negotiations with the leaders of Massilia when Domitius reached the city. These negotiations had come to something of a standstill, with the Massiliotes stating that as both Caesar and Pompey had helped the city in the past they couldn't chose between the two rivals, and would instead close their gates to both factions.
Domitius's arrival at the head of his small force seems to have tipped the balance in favour of Pompey. He was accepted as governor of the city and put in charge of its defence. The city's fleet was used to capture passing merchantmen and bring their cargoes into the city, the walls were repaired and armouries were opened in the city.
The hostility of Massilia threatened Caesar's lines of communication with Spain, where he had already decided to campaign next. In response he led three of his legions to the city, while three legions that were already at Narbo were sent into the Pyrenees. A fleet of twelve ships was built at Arelas and placed under the command of Decimus Brutus. The three legions outside Massilia were placed under the command of Caius Trebonius, leaving Caesar free to follow his armies into Spain.
Two naval battles were fought outside Massilia. In the first a Massiliote fleet of seventeen ships of war and a number of smaller ships attacked Caesar's smaller fleet. Brutus's legionaries outfought their opponents, capturing or sinking nine enemy ships.
The second naval battle involved the Massiliote fleet and a squadron sent by Pompey. The Roman fleet was also larger than in the first battle, having absorbed some of the captured ships. This second battle ended in an even clearer Roman victory - only one ship returned to Massilia, with the rest of their fleet sunk, captured or fleeing to Spain.
Meanwhile Trebonius embarked on a regular siege. He built works in two places - once close to the harbour and one against the part of the city between the Rhone and the sea. These works included a mount 80 feet high, but many of the other siege works were destroyed by artillery fire from within the town. This forced Trebonius to protect his siege works with heavy timbers, slowing down progress. A large tower was built, and a covered way built out towards the town walls. This allowed the Romans to begin to undermine the wall, and a tower was soon close to collapse.
At this point the inhabitants of the city asked for a truce, to run until Caesar returned from Spain. Trebonius agreed to this, partly because Caesar had left orders that the city was not to be taken by storm - he didn't want to be responsible for the destruction of a great Greek city and a key prop of Roman power in southern Gaul.
According to Caesar this truce only lasted for a few days before the Massiliotes broke it, catching the Romans by surprise. Their siege works were almost totally destroyed under cover of a storm. This good work was very quickly undone by the Romans, who rebuilt their siege works, this time largely in brick, and morale in the city fell to a low ebb, and talk of surrender spread. Domitius Ahenobarbus took advantage of this warning and escaped from the city by sea.
His escape came just in time. Caesar returned from Spain just at the defenders were about to surrender. The Massiliotes brought their weapons, shops and money out of the city. Caesar placed two legions in the city as a garrison. The city also lost most of its surrounding territory and dependent towns, but was otherwise left alone. Caesar was now free to return to Italy, where he became Dictator, before moving on to Greece to face Pompey and the most important of his opponents.