The battle of Nola (Summer, 89 BC) was a series of encounters in which Sulla defeated a Samnite attempt to raise his sieges of Herculaneum and Pompeii (Social War).
In 90 BC Nola (east of Naples) had fallen to the Samnite leader Gaius Papius Mutilus. He had then moved south and captured Stabiae, Surrentum, Salernum and Niceria, all in the area to the south of Pompeii. He then moved back north, and attempted to capture Acerrae, between Naples and Nola, but we don't know how this siege ended. We also don't know when or why Pompeii and Herculaneum joined the Italian allies, but by 89 BC they were clearly in Italian hands.
In 89 BC Sulla was given command of the Roman forces in Campania. He moved south, and camped in the hills near Pompeii, while his armies besieged Herculaneum and Pompeii. Herculaneum probably fell on 11 June, and Pompeii some time after this, but we don't know where the battle of Nola fits into this timeline.
Our only detailed account of the battle comes from Appian. While Sulla was camped in the hills, a Samnite relief army under Lucius Cluentius approached, and camped only three stades from Sulla's own camp (well within a mile). At this point Sulla's army was divided, with foragers out of camp, but despite this Sulla decided to attack immediately. This first attack ended in failure, but after his foragers returned Sulla counterattacked and forced Cluentius to pull back and camp further away.
We now reach a rare mention of the Gauls during the Social War. Cluentius received some Gallic reinforcements, and decided to return to the attack. Just before the battle, a giant Gaul stepped forward and challenged any Roman to single combat. A short Mauritanian soldier took up the challenge, and killed the Gaul. Cluentius's Gauls fled after the death of their champion, leaving a gap in Cluentius's line. The rest of his army broke and fled north towards Nola.
Sulla killed 3,000 Italians during the pursuit, but the real victory came outside the walls of Nola. The town was still in Italian hands, but its defenders were only willing to open one gate, to reduce the chance that Sulla could rush in behind the refugees. As a result 20,000 more Italians, including Cluentius, were killed outside the walls.
Our other sources provide very few details of this battle. Eutropius mentions that Sulla defeated Cluentius so easily that he only lost one man from his own army. This is almost certainly an exaggeration, but if Cluentius's army fell apart at the start of the final battle then the Roman casualties probably won't have been very high.
Orosius mentions the battle in the context of the murder of Postumius Albinus, one of Sulla's legates, by his own men. Sulla decided not to punish the soldiers, and instead said that 'civil bloodshed could be atoned for only by shedding the blood of the enemy'. His men realised that they would have to redeem themselves in battle, and killed 18,000 Samnites, including Cluentius. The periochae of Livy only reports that Sulla defeated the Samnites in battle and expelled them from two of their camps - possibly the two camps mentioned in Appian in the context of this battle.
Cicero probably mentions this battle in his work On Divination. Sulla was offering sacrifices in front of his headquarters near Nola when a snake appeared from beneath the altar. The soothsayer Gaius Postumius begged Sulla to proceed with his march at once, and Sulla captured a strongly fortified Samnite camp outside Nola. According to Plutarch Cicero actually served under Sulla during the Social War, so he might have been present at this battle (In his own work Cicero mentions serving under Pompey Strabo, but there is no reason why he couldn't have served under both).
After winning this victory Sulla probably turned back to complete the sieges of Herculaneum and Pompeii. He then advanced east from Nola to besiege the Hirpini town of Aeclanum. He then defeated the Samnites after outwitting Papius Mutilus and went on to capture the location of their common council at Bovianum.