Leaving Mark Antony in charge of Italy, Caesar set out west for Spain. En route, he started a siege of Massilia when the city barred him entry and came under the command of the aforementioned Domitius Ahenobarbus. Leaving a besieging force, Caesar continued to Spain with a small bodyguard and 900 German auxiliary cavalry.

After the siege had begun, Ahenobarbus arrived in Massilia to defend it against the Caesarian forces. In late June, Caesar's ships, although they were less skilfully built than those of the Massiliots and outnumbered, were victorious in the ensuing naval battle.

Gaius Trebonius conducted the siege using a variety of siege machines including siege towers, a siege-ramp, and a "testudo-ram". Gaius Scribonius Curio, careless in adequately guarding the Sicilian Straits, allowed Lucius Nasidius to bring more ships to the aid of Ahenobarbus. He fought a second naval battle with Decimus Brutus in early September, but withdrew defeated and sailed for Hispania.

At the final surrender of Massilia, Caesar showed his usual leniency and Lucius Ahenobarbus fled to Thessaly in the only vessel that was able to escape from the Populares. Afterwards, Massilia was allowed to keep nominal autonomy, due to ancient ties of friendship and support of Rome, along with some territories while most of its empire was confiscated by Julius Caesar.