The 'battle' of Teanum (83 BC) saw Sulla win over almost the entire army of the consul Scipio Asiaticus, winning a bloodless victory over the second of the two consular armies that had been sent against him as he advanced towards Rome (Sulla's Second Civil War).
Although Sulla's invasion had been expected since at least 84 BC, he was allowed to make an unopposed crossing from Greece to Brundisium, and was able to reach Campania before encountering any resistance. As he advanced Sulla was joined by Metellus Pius, the most senior member of the Roman aristocracy to come over to him in 83 BC, and by the young Pompey. The two consuls for the year, Gaius Norbanus and Lucius Cornelius Scipio Asiaticus, had raised a powerful army (250 cohorts of 500 men or 125,000 men), but they appear to have been caught out by Sulla's appearance, and the army had to be sent south in detachments. Norbanus commanding the leading troops with Scipio following on behind.
The first battle came at Casilinum, on the Volturnus, where Sulla defeated Norbanus and forced him to take refuge a short distance to the south in Capua. Although Norbanus suffered heavy casualties in this battle, at least part of his army remained intact and under his control.
Sulla now turned north to deal with Scipio's army, which Plutarch gives as being forty cohorts strong (about 20,000 men), but Appian describes as downhearted and longing for peace. Sulla moved north to Teanum (Teano), about ten miles nmorth of Casilinum. Scipio advanced south towards Sulla's position, but then accepted Sulla's offer of negotiations. Scipio took hostages in an attempt to guarantee that Sulla wouldn't take advantage of the meetings, and then moved 'down to the plain', from the higher ground to the north of Teanum (the town is on the south-eastern slopes of Rocca Monfina, an extinct volcano.
The first meeting was between three representatives from each side, and what was said or who was present wasn't recorded. However it was significant enough for Scipio to send Sertorius to Norbanus in Capua to discuss them. On the way to Capua Sertorius seized Suessa (Sessa Aurunca), which had submitted to Sulla. This can't have been accidental (or done on a whim) as Suessa is due west of Teanum, way off any obvious route between Teanum and Capua. Sulla complained about this action, and Scipio responded by returning his hostages (Appian suggests that this was either because he had either been involving in planning the move, or was uncertain how to respond to an unauthorised attack). Scipio's men blamed Scipio for breaking the terms of an armistice. At the same time Sulla's men had been circulating in Scipio's camp, slowly winning over his men.
Scipio's men now agreed to come over to Sulla if he would move near to their camp. Plutarch has Sulla approaching Scipio's force with twenty cohorts (10,000 men), at which point Scipio's forty cohorts greeted them and switched sides. Both Appian and Plutarch report that Scipio was left in his tent, unaware of what was being planned until it had happened (Appian puts his son Licius Scipio with him).
Sulla attempted to convince Scipio to change sides, but he refused. At this point Sulla was portraying himself as a moderate, generous foe, and so he let Scipio and his son go. Scipio may have made an agreement not to fight against Sulla again, which he later broke, for at the start of his proscriptions Sulla announced that he was going to punish anyone who had acted against him from the day when Scipio broke his agreement.
Although the desertion of Scipio's entire army had greatly strengthened Sulla's hand, he still wasn't strong enough to risk an attack on Rome. He attempted to open negotiations with Norbanus, but he refused to even acknowledge Sulla's messages, not wanting to share Scipio's fate. Both sides then advanced along different roads, presumably in the direction of Rome, but Sulla stopped before getting too close to the city. The consuls still had the support of most of Italy, and were able to raise fresh troops, and for the rest of the campaigning season of 83 BC both sides focused on gaining support and securing their control of the areas already in their hands.