The battle of Dyrrhachium (20 May 48 BC) was the most serious setback suffered in person by Caesar during the Great Roman Civil War and saw Pompey break out of a blockade south of Dyrrhachium on the eastern coast of the Adriatic.
At the start of January 48 BC Caesar managed to cross the Adriatic at the head of seven legions, slipping past Pompey's blockading fleets. Caesar's transport ships were caught and thirty destroyed on the return journey, leaving him isolated and outnumbered. Pompey moved west from his training camps in Macedonia, and just managed to prevent Caesar from occupying the sea port of Dyrrhachium.
In February, after a standoff that lasted for several months, Mark Antony finally managed to get to sea with reinforcements, but his fleet of sailing ships was forced north, past both Caesar and Pompey, eventually landing to the north of Dyrrhachium. Pompey attempted to stop the two enemy armies from joining up, but failed, and retreated back to a position in the territory of Dyrrhachium. After another brief standoff here Caesar decided to try and seize Dyrrhachium in a surprise attack. This plan almost succeeded. Caesar's men reached the outskirts of the city first, but Pompey was not far behind. He took up a position a few miles further south down the coast, at a place called Petra, where there was a small anchorage.
Over the next few weeks another rather more elaborate standoff developed. Both commanders build strong field fortifications, until two lines of forts and connecting walls stretched for fifteen miles around Pompey's beachhead. He also retained control of Dyrrhachium.
This was a most unusual siege. Pompey had the larger army and the best access to supplies, which came to him by sea. Caesar had access to the surrounding countryside, but this had been stripped clear of supplies by Pompey's men. Gradually the situation changed. Pompey struggled to get fodder for his horses, while all around Caesar the crops began to ripen.
Pompey decided to try and break Caesar's blockade. Sadly his first attempt falls in a gap in Caesar's account of the fighting, but appears to have involved a plan to draw Caesar to Dyrrhachium while three attacks were made on the lines. This attack failed, and Pompey's second (and better documented plan) was a rather simpler affair.
Pompey was aided by two Gallic leaders, sons of the ruler of the Allobrogians. They had been accused of keeping all of their men's pay, and after receiving a private rebuke from Caesar they went over to Pompey. They provided him with details of Caesar's defences, and revealed that the southern end of the line was a weak spot. This was the point furthest from Caesar's camp, which was at the northern end of the lines.
At the southern end of the line Caesar had constructed double lines, with a ten-foot high rampart facing north and a smaller rampart facing south, with a 600ft gap between the two. The southern rampart was to guard against any possible amphibious assault by Pompey's men. Caesar had planned to build a cross-wall between the two camps, but this hadn't yet been built. The commander of the southern end of the line, Lentulus Marcellinus, was in poor health, and so Fulvius Costhumus had been sent to assist him. Next in line was Mark Antony.
Pompey decided to throw a very strong force at this weak point in the defences. Sixty cohorts of infantry (the equivalent of six full legions) were taken from the lines, and were supported by a large number of ship-borne light infantry and archers.
Pompey's attack began at dawn. While the legions launched a frontal assault on Caesar's inner wall, the lightly armed troops and archers attacked from the sea. Pompey then landed between the two walls, and his men attacked both of Caesar's lines from the rear. This caused a panic, which Marcellinus was unable to stem. A number of Caesar's men were killed in the crush as they fled, and Pompey was soon approaching Mercellinus's camp.
Mark Antony now intervened, attacking with 12 cohorts. This stopped the Pompeian advance and gave Caesar time to arrive on the scene with reinforcements, but it was already clear that the blockade had been broken. Pompey was camped on the coast, and had regained his access to the surrounding countryside.
Caesar ordered the construction of a new fortified camp opposite Pompey's new position, but he was soon given a chance to launch a counterattack. Some days earlier Pompey had occupied a small camp that had been abandoned by Caesar, enlarged the defences, but then abandoned it. He now sent a legion to reoccupy this camp. Caesar responded by leading 33 cohorts to attack this position. Pompey's legion was forced back, and a more serious defeat was only prevented when some of Caesar's men followed a wall that led from the camp to a nearby river, thinking it was the outer rampart of the fort.
This gave Pompey the time to organise his own counterattack. Some of his men made a stand at one of the camp gates, and forced Caesar's cavalry to flee. Caesar's right wing, seeing the cavalry retreat, turned back and retreated in some chaos to avoid being trapped. A number of men were trampled while attempting to cross a rampart and ditch. The retreat developed into something of a rout, which even Caesar was unable to prevent. Fortunately for Caesar some of his men remained firm, and defended the gates into the fort long enough to prevent Pompey from taking advantage of this unexpected victory.
Caesar reported his casualties in the fighting as 960 rank and file, including four named Roman knights and thirty-two military tribunes and centurions.
In the aftermath of this defeat Caesar retreated to Apollonia, but he then decided to leave his position on the west coast and advance across the Balkans towards a second army that was approaching from the east. On his way he captured the city of Gomphi, before a second deadlock developed around Pharsalus, the site of the decisive battle of the campaign.