The siege of Dyrrhachium (March-May 48 BC) was the first direct confrontation between Caesar and Pompey during the Great Roman Civil War, and ended as a victory for Pompey after he broke through Caesar's siege lines.
In the first few weeks of the Civil War Caesar had advanced down the Italian peninsula with such speed that Pompey and the Senate had been forced to abandon Italy and retreat to Greece, where they were able to gather their strength and build up a powerful army. By the start of 48 BC Pompey had nine full strength legions under his direct command, and another two veteran legions coming from Syria. His biggest problem was that his men lacked the combat experience of Caesar's veterans, many of whom had fought with him in Gaul.
Caesar had twelve under-strength legions in Italy, but he could be confident that they would win any pitched battle against Pompey's forces. Caesar's main problem was that Pompey's fleets, commanded by L. Bibulus, controlled the Adriatic, making it dangerous to attempt to sail from Brundisium to Greece, but the land route through Illyria would have taken too long.
Caesar was also suffering from a shortage of shipping. According to his own account of the campaign his men insisted on leaving their slaves and baggage behind, and on 4 January 48 BC he set sail at the head of seven legions. On the following day, having slipped past Pompey's fleets, he landed somewhere to the south of the Acroceraunian mountains,
Bibulus arrived on the scene just too late to intercept Caesar, but he was able to catch the fleet as it returned to Italy to pick up the next wave of troops. Thirty ships were taken and set of fire with their crews still on board. He committed a similar atrocity a little later when a single ship fell into his hands.
After landing Caesar sent one of Pompey's captured supporters to him to ask for a peace conference, and then moved north, capturing Oricum, then Apollonia, two important ports on the coast of Epirus (now southern Albania). This just left Dyrrhachium (originally Epidamnus, now Durres in Albania) in Pompey's hands. Alerted by the message from Caesar, Pompey pushed his troops on a forced march and just managed to block Caesar's route north, retaining control of Dyrrhachium.
At this point the advantage was with Pompey. He had easy access to supplies, with control of the sea and a much larger army than Caesar, but he was unwilling to risk a battle, even though there was always a chance that Caesar would receive reinforcements.
The standoff lasted until late February, when Mark Antony finally managed to get to sea with four legions, some slingers and 800 cavalry. His fleet of sailing ships was blown past Caesar at Apollonia and Pompey at Dyrrhachium, and eventually reached land at Nymphaeum (now Shengjin in Albania, previously known as S. Giovanni di Medua).
Caesar now had eleven legions, but they were split in two, with Pompey between them. Pompey attempted to ambush Antony, but his movements were reported by local supporters of Caesar. Antony halted his march for one day to all Caesar to arrive, and rather than risk fighting two armies at the same time Pompey retreated to Asparagium, in the territory of Dyrrhachium. Caesar followed and offered battle, but when Pompey refused to fight yet again Caesar decided to try and make a dash for Dyrrhachium, using an obscure and round-about route in the hope that Pompey wouldn't realise where he had gone.
The plan almost worked. Caesar's men arrived outside Dyrrhachium while Pompey was still some distance away, but the lead was not enough. Pompey established himself on some higher ground at Petra, a nearby anchorage to the south of Dyrrhachium. This prevented Caesar from concentrating on a formal siege of Dyrrhachium, and instead forced him to deal with Pompey's main army.
Both commanders quickly began to build field fortifications - Caesar in an attempt to blockade Pompey and Pompey in order to secure as large an area as possible. Eventually the lines ran for fifteen miles, and enclosed a large area along the coast. Caesar and Pompey both placed their main camps at the northern end of the siege works, with Pompey's at or close to Petra. The two lines of fortifications were created in something that would appear to have been similar to the 'race to the sea' of 1914, with Caesar attempting to reach the sea as near as possible to Petra, and Pompey expanding his lines to stop him.
Pompey still had command of the sea, and so Caesar was forced to build a double line of fortifications, to guard against the possibility of an amphibious assault. He was also forced to detach a number of legions from the main army to try and find supplies, and so the southern end of the line was incomplete, with no cross walls linking the inner and outer lines.
Once the two lines of fortifications had been completed the supply situation gradually began to turn against Pompey. Although food for the men could easily reach his beachhead, fodder for the horses was harder to find, and they soon began to sicken. After a siege that probably lasted for around six weeks, Pompey decided to try and break through Caesar's lines.
The first attempt is made obscure by an unfortunate gap in Caesar's Commentary on the Civil War. Caesar was tricked into leaving his main camp by a report that Dyrrhachium was about to be betrayed to him. P. Sulla was left in charge of the lines, which were attacked in three places. The most serious attack saw a single cohort hold off an attack by several legions before reinforcements arrived, but all three attacks eventually failed.
The second attack (battle of Dyrrhachium) took advantage of inside knowledge that came to Pompey from two Gallic deserters, who informed him that the southern part of Caesar's line was vulnerable. This attack was more successful, and Caesar only narrowly avoided a serious defeat. In the aftermath of this defeat Caesar decided to change his plan, and move east, away from the coast and Pompey's fleet. This campaign ended with the two armies facing each other at Pharsalus, the site of the decisive battle of the war.