The battle of the Teanus River (88 BC) was the last major battle of the Italian Social War, and ended with the death of Quintus Poppaedius Silo, one of the most able of the Italian commanders.
This is one of the more poorly documented battles of the war, and our various accounts disagree on key elements, in particular the Roman commander.
Appian tells us that Metellus Pius, then serving as praetor, defeated the Apulians in battle. Poppaedius was killed in the battle, and the survivors of his army slowly came over to Metellus.
The summary of Livy tells us that the after the Italians were defeated by legate Aemilius Mamercus the Marsian leader Poppaedius was killed in battle.
Orosius gives us a location - the Teanus River, a different Roman commander - Sulpicius, one of Pompey Strabo's legates and tells us that the Italian leaders Poppaedius and Obsidius were killed in the battle.
The location of the Teanus River is now unknown. If Appian is right, and the battle involved the Apulians, then it was probably close to Teanum Apulum, near San Paolo di Civitate in Apulia. This would place it in the province of Foggia in Apulia, in eastern Italy, towards the western end of the flatter area east of the mountains.
Metellus Pius is the most likely Roman commander during this battle. In 87 BC, when Sulla's enemies attacked Rome (Sulla's First Civil War), he was still campaigning against the Samnites. Aemilius Mamercus was probably Mamercus Aemilius Lepidus Livianus, consul for 77 BC, so may have been Metellus's legate in 89 BC. Sulpicius's presence is less easy to explain - Pompey Strabo spent most of the war campaigning in Picenum, further to the north on the Adriatic coast, but if this battle took place late in 89 or in 88, after the fall of Asculum, then he might have send part of his army south-east down the coast to help hunt down some of the last surviving Italian leaders.
Julius Obsequens, in his Book of Prodigies, tells us that Poppaedius Silo held a triumph in Bovianum in 88 BC (dating it to the consulship of Sulla and Quintus Pompeius Rufus), after recapturing the city from the Romans (it was captured by Sulla in 89 BC). This was an ill omen, because a triumph was normally held in the conquering city, not the conquered city, and Silo died in his next battle. However if Silo did recapture Bovianum, then it could be seen as both the conquered and conquering city. In either case, this dates the battle of the Teanus River to 88 BC.
A date of 88 BC would fit with the overall flow of the war. By then most of the Italian rebels had surrendered, and only the Samnites and Lucanians were still holding out. In 89 BC the praetor Gaius Cosconius had defeated the Samnites in a battle close to Canusium, and gone on to regain control of much of the surrounding area, but the defeated Samnite leader Trebatius had taken refuge in Canusium. Silo could have been attempting to rescue Trebatius, and then restore the situation in Apulia, which would at least have given the Samnites and Lucanians control of most of the southern tip of Italy. His defeat ended any chance of holding on in Apulia, and Sulla was free to use the port of Brundisium when he took his army to the east to fight Mithridates VI of Pontus (First Mithridatic War) later in 88 BC.