The siege of Brundisium (49 BC) saw a brief confrontation between Pompey and Caesar at the start of the Great Roman Civil War, before Pompey escaped to Epirus.
At the start of the war the Senate gave Pompey the authority to defend the Republic against Caesar, and confidently believed that he would be able to raise armies with which to defend Italy before Caesar could move his veteran legions from Gaul. However, early in 49 BC Caesar crossed the Rubicon, then seen as the Italian border, with one legion. He advanced south at great pace, gathering support as he went. Pompey quickly realised that he couldn’t defend Rome, and decided to retreat across the Adriatic to Epirus, where he could raise a massive army, made up of supporters of the Senate and the forces of his client kings across the east.
Before the outbreak of open warfare Caesar had moved to Ravenna, on the Adriatic coast but still within his province. From there he advanced down the east coast, capturing Corfinium (due east of Rome) after a short siege. His army continued to expand as he moved south. By the time he reached Brundisium, Caesar had six legions - three veteran and three newly raised.
By the time Caesar arrived outside Brundisium most of the Senate’s army, along with the two consuls for the year, had already departed for Dyrrhachium, but Pompey with twenty cohorts was still in the port. Pompey was actually waiting for the return of the transport ships and was planning to join the main army, but Caesar was worried that he intended to try and hold on to Brundisium to keep a foothold in Italy and keep control of the Adriatic.
In order to prevent this, Caesar began work on moles on either side of the entrance to the port, which was at the head of a narrow bay. The central part of the channel was too deep for this to work, and so Caesar closed this gap with floating wooden structures topped with earth and defended with fascines and a parapet. Some of these giant rafts carried two storey towers. Pompey reacted by building three storey towers on some of the largest ships found in the port, and used them to attack the works on the moles and floats. A series of skirmishes took place between Caesar’s men on the barrier and Pompey’s on the ships.
At the same time Caesar kept on making attempts to win over Pompey, sending a series of messengers into the besieged city. Pompey’s answer was that he couldn’t make peace without the permission of the consuls, who were no longer present.
Caesar had been working on his barrier for nine days when Pompey’s transport fleet returned. Pompey decided to evacuate the city as quickly as possible. First he had the gates walled up, built barricades or ditches across most of the streets (leaving two leading to the port open but fortified). He then ordered most of his men to embark, leaving a rearguard of slingers and archers to man the walls. Once the main force was embarked, they were to move onto small ships and escape.
The evacuation went well, despite the people of Brundisium apparently taking against Caesar and signalling him when it began. Caesar’s men were easily able to capture the walls and enter the port, but then had to take a long route to the port to avoid Pompey’s ditches and traps. This allowed most of Pompey’s fleet to escape, with only two trapped against the new moles.
Caesar was now faced with a dilemma. The only way to end the war quickly would be to follow Pompey and defeat the Senate’s army somewhere in the Balkans, but there were no longer any ships on most of the Italian coast. He thus had to summon ships from the northern Adriatic, Sicily and Gaul, which would have caused a long delay and left Caesar’s army inactive for some time. In order to avoid this, Caesar decided to turn the other way, and make a daring attack on Spain, where Pompey’s own army was waiting. After a brief visit to Rome to establish his authority (and demonstrate his famous mercy), Caesar began an impressively rapid Spanish campaign, defeating Pompey’s best army at Illerda in the summer of 49 BC, knocking away one of the key hopes of the Senatorial side.