The battle of Teanum Sidicinum (90 BC) saw the Roman consul Lucius Julius Caesar suffer a defeat at the hands of the Samnite leader Marius Egnatius (Social War), probably during an attempt to lift the siege of either Aesernia or Acerrae.
Caesar was the commander of the Roman forces in Campania, where he was faced with a series of problems. The Italian rebels had captured a number of towns across Campania, including Nola to the north-east of Naples. Caesar had already suffered one defeat, at the hands of the Marsian leader Vettius Scato, who then besieged Aesernia, on a key transport link across the Apennines. Further south Gaius Papius, the captor of Nola, was besieging nearby Acerrae. Caesar's first attempt to lift the siege of Acerrae had failed after Papius attacked his camp.
Caesar received reinforcements, bringing his army up to 30,000 infantry and 5,000 cavalry. We next encounter him as he was passing through a rocky defile somewhere in the vicinity of Teanum (modern Teano), thirty miles to the north of Naples, and equally distant from both besieged cities. Our sources don't say what he was attempting to achieve - Orosius mentions a defeat at Aesernia, which may have been the earlier defeat at the hands of Scato, while Appian only tells us what he did next.
As he was passing through the rocky defile, Caesar's army was attacked by Marius Egnatius, one of the Samnite leaders. Caesar's army was pushed back into the valley, and up against a river that could only be crossed on a single bridge. This suggests that he was operating in the mountains to the north of Teanum, perhaps in an attempt to lift the siege of Aesernia. Caesar, who was ill at the time, narrowly escaped from the trap, having to be carried to safety in Teanum on a litter. Most of his army was lost, and many of the survivors abandoned their equipment in the rout.
Once he was in Taenum, Caesar rearmed the survivors. He then received reinforcements, and marched south to try and raise the siege of Acerrae. A standoff developed outside Acerrae, with neither side willing to risk a battle. Unfortunately that is where our knowledge of the siege ends. Caesar had to return to Rome to hold the elections for 89 BC, and we don't have any record of the city being recaptured by the Romans.
Caesar's most important contribution to the Roman war effort came back in Rome, where towards the end of his consular year he put forward the Lex Iulia de Civitate Latinis Danda, which offered Roman citizenship to all Latin communities and all Italian communities that hadn't revolted. This stopped the war spreading into Etruria, and reduced its impact in Umbria, and led to wider concessions in the following year that largely removed the reason for the conflict.