The siege of Aesernia (90 BC) was a success for the Italian allies, and saw them capture the Latin colony of Aesernia after a lengthy siege (Italian Social War).
Aesernia (modern Isernia) sits on a ridge between the Carpino and Sordo valleys in the Apennines, on the road from Corfinium (the capital of the Italian allies at the start of the war) to Beneventum. If the Romans could hold onto it, it would have made communications between the Samnite and Marsic groups of rebels much more difficult.
Like so many events of this war, the sources for the siege of Aesernia pose some problems.
Appian provides a brief account of the siege. P. Vettius Scato defeated the consul L. Julius Caesar, and then went on to besiege Aesernia, which held out for some time. The Roman commanders at Aesernia, L. Scipio and L. Acilius, escaped disguised as slaves. After some time the city surrendered due to hunger. In 89 BC the Samnite leader Gaius Papius Mutilus took refuge in Aesernia, after he was defeated by Sulla.
A fragment of Diodorus provides two more details. As food began to run out, the defenders of the city expelled the slaves, who were welcomed and treated well by the besieging forces. The defenders were still forced to eat dogs and other animals.
The summary of Livy only says that the city fell to the Samnites, along with its commander Marcus Marcellus.
Orosius tells us that the Roman citizens and soldiers were hard pressed by a very close siege. Sulla then attacked with 24 cohorts, won a great battle and saved the city.
According to Frontinus Sulla was surprised in a defile near Aesernia by an army led by Duillius. He asked for a peace conference, but the talks failed. Sulla realised that the enemy had dropped their guard because of the talks, and escaped under the cover of darkness, leaving a single trumpeter in the camp to sound the watches as normal. Sulla was then able to escape with his baggage and siege train.
Strabo reports that the city was destroyed in the war, presumably after it was recaptured by the Romans.
These separate accounts could be combined to produce several different versions of the siege (and indeed to support the idea that there were two sieges - one before and one after Sulla's relief effort).
Perhaps the most likely course of events is that the siege began after Vettius Scato defeated Caesar. Food began to run short, and the Roman commanders at the start of the siege, L. Scipio and L. Acilius, escaped when the slaves were expelled. Sulla was sent to lift the siege, ran into the otherwise unknown Duillius, and managed to escape with his army intact. He may have managed to briefly lift the siege, getting supplies and a new commander (Marcus Marcellus) into the city, but he was then forced to withdraw. The siege resumed, and the defenders were eventually forced to surrender by hunger. Sulla's great victory, reported by Orosius, probably belongs to the campaign of 89 BC, when Papius Mutilus was forced to take refuge in the city.
It is also possible that Scipio and Acilius escaped after the failure of Sulla's relief expedition, leaving Marcellus as the most senior Roman in the city.
After the fall of Aesernia it remained in Italian hands into 89 BC, and was used as Papius Mutilus's headquarters after Sulla's successes. The rebels had already been forced to abandon Corfinium, and Sulla soon captured their second capital, at Bovianum, so Aesernia probably became their third capital.
The city was retaken by the Romans, and was destroyed, suggesting that there might have been a second, Roman, siege that is unmentioned in our sources (which get distracted by the events of Sulla's First Civil War). When the city fell to the Romans isn't clear - the Samnites remained in arms against the Romans after end of the Social War, allied themselves with Sulla's enemies in Rome, and were only finally defeated after Sulla's Second Civil War (83-82 BC), so the city might have remained in Samnite hands for almost ten years.