The siege of Asculum (90-89 BC) was one of the longest sieges of the Social War, and eventually saw Pompeius Strabo capture the city, after a siege that may have lasted for over a year.
The city of Asculum was in the centre of southern Picenum. The Social War had broken out in the city in 91 BC, after a visiting Roman magistrate, one of many sent out to investigate rumours of trouble, inflamed the locals and was killed. This was followed by a massacre of all the Romans in the city, making Asculum an inevitable target for the Romans.
Command of the army sent to capture Asculum was given to Gnaeus Pompeius Strabo (father of Pompey the Great, the triumvir), who had sizeable landholdings in Picenum. At first his campaign didn’t go well. Three of the Italian commanders combined their armies, and defeated Pompeius at Mount Falernus, somewhere to the north of Asculum. Pompeius had to flee east to safety in Firmum, where he was besieged for some time. Eventually he managed to defeat the besieging army, and its survivors fled south to Asculum, pursued by Pompeius, who was finally able to besiege the city.
Appian's account of the siege is frustratingly incomplete. The start of the siege is immediately followed by an account of a relief effort, led by Vidacilius, one of the victorious generals at Mount Falernus, who was from the city. He led eight cohorts (4,000 men) to the city, sending a message ahead ordering the defenders to make a sortie when he attacked. The message got through, but the defenders were afraid to risk this attack. Vidacilius was still able to break into the city, but he quickly realised that it couldn’t be saved. He killed his opponents in the city, then held a feast at which he committed suicide. The Roman commander during this phase of the siege isn't mentioned.
After the death of Vidacilius Appian tells us that the proconsul Sextus Caesar (consul in 91 BC) was given command of the siege, but died of disease and was replaced by Gaius Baebius. Pompeius stood for election as one of the consuls of 89 BC, which would have required him to return to Rome, so Sextus may have been given the command at that stage. Frustratingly this is the last mention of Asculum in Appian.
Pompeius was elected as one of the consuls for 89 BC. His first victory as consul came over an army that was being sent from the Adriatic coast to try and support a possible revolt in Etruria. Pompeius intercepted this army and killed 5,000 of the Italians. The rest were forced to retreat across the mountains in mid-winter and half of them died. This may be the same battle mentioned in Livy as the Consul Pompeius defeating the Marsians in an open battle.
Orosius records two battles taking place on the same day during Pompeius's siege of Asculum. The first was between Pompeius and the Marsi, under a general called Fraucus. Pompeius killed 18,000 of the Marsi, including Fraucus. Another 4,000 fled to the summit of a mountain where they were killed by exposure. On the same day the Romans also defeated the Picentes, and their leader Vidacilius committed suicide.
This may be a slightly confused version of the events mentioned in Appian - Pompeius's victory over the force heading to Etruria and Vidacilius's relief effort. Livy reports a victory for Consul Pompeius over the Marsians in an open battle, again possibly the defeat of the army heading for Etruria.
Pompeius is recorded as campaigning elsewhere during his year as consul. Pompey has him taking the surrender of the Marsians, Marrucini and Vestini, all areas to the south of Asculum, which would explain his absence from the siege.
Velleius Paterculus mentioned a battle between 75,000 Roman citizens and 60,000 Italians, fought near Asculum, but this comes in a discussion of Cina's attack on Rome, and he provides no context for this battle.
Orosius provides an account of the fall of Asculum. This comes after the battle of the Teanus River, which might have been in 89 or 88 BC. Pompeius entered the city, and had all of the prefects, centurions and leading men of the city beaten and beheaded. He then sold off the slaves and ordered the remaining inhabitants to leave the city.
If the events in the periochae of Livy are in the same order as in the lost full text, then Pompeius was a proconsul when he captured Asculum, placing the fall of the city in 88 BC. By this point most of the other rebels had surrendered or been defeated, allowing the proconsul to return to his original task. However this would place the fall of Asculum after Pompeius's triumph, which probably took place on 27 December 89 BC, at the end of his time as consul. The general consensuses is that the siege ended in November 89 BC.