The siege of Acerrae (90 BC) saw the Italian rebels defeat a series of Roman attempts to lift the siege, but frustratingly we don't actually know how it ended (Italian Social War).
Acerrae is a town about nine miles to the north-east of Naples. In 90 BC the nearby city of Nola (eight miles to the east) fell to the Samnite leader Gaius Papius Mutilus through treachery. 2,000 Roman soldiers captured in the city agreed to change sides, although their officers refused and were starved to death.
Papius captured a series of towns in the area, including Stabiae, Salernum and Surrentum, and gained another 10,000 infantry and 1,000 cavalry from the local area. He then began a siege of Acerrae.
The Roman attempts to lift this siege were led by Lucius Julius Caesar, one of the consuls for 90 BC. For his first attempt he gathered a force of 10,000 Gallic infantry, supported by Numidian and Mauritanian cavalry, a sign of the wider resources available to the Romans in this war. However the multinational nature of his force also proved to be a weakness. Papius had captured Oxynta, a son of the defeated Numidian king Jugurtha, and paraded him to Caesar's army. Many of the Numidians deserted Caesar, on the grounds that Oxynta had a greater call on their loyalty. In order to stop this from spreading, Caesar was forced to sent the rest of his Numidians home.
Papius then decided to attack Caesar's camp. He managed to force a breach in the palisade that surrounded the camp, but Caesar led his cavalry out of another gate, and attacked Papius from the rear. According to Appian he killed around 6,000 of Papius's men, but Caesar's situation was clearly still rather precarious, and soon after repelling this attack he withdrew from the area.
Caesar next appeared at the head of a 35,000 strong army, but this was defeated near Teanum, some way to the north of Acerrae. After escaping to Teanum, Caesar rearmed the survivors of the defeat, and received fresh reinforcements. He led this new army south to make another attempt to lift the siege of Acerrae.
Caesar and Papius camped opposite each other, somewhere outside the city, but neither of them was willing to risk a battle. Frustratingly, after this Appian never mentions the siege again, so we don't know how it ends. Caesar eventually had to return to Rome to hold the elections for 89 BC (and to sponsor the Lex Iulia de Civitate Latinis Danda, the first in a series of Roman concessions that helped end the war). Papius isn't mentioned again until the following year, when he suffered a defeat at the hands of Sulla. Acerrae isn't mentioned during Sulla's campaign in the area in 89 BC, but that doesn't really tell us anything.