The battle of Placentia (82 BC) saw the Sullan commander Marcus Lucullus defeat a Marian army in the Po Valley, helping to end their command of Cisalpine Gaul.
Marcus Lucullus's activities in the Po are only referred to in fragments, and it isn't clear if the different sources are talking about one or two battles. M. Lucullus was the brother of the more famous L. Lucullus of the Third Mithridatic War, who spent the period of this civil war in the east.
According to Appian Lucullus 'won a victory over another body of Carbo's forces near Placentia', quite late in the war, after Carbo's attempt to ambush Metellus Pius at Faventia had failed.
Plutarch and Velleius Paterculus both report a battle at Fidentia. Velleius Paterculus has it happening before the battle of Sacriportus, so very early in the campaign of 82 BC, but this comes in a list of three battles of which one definitely happened after the battle of Sacriportus (battle of Faventia), and includes an uncertain reference to a battle at Clusium.
Plutarch's account comes as part of a series of miraculous signs that encouraged Sulla and his men. Lucullus had only 16 cohorts (8,000 men), and faced 50 cohorts of the enemy (25,000 men) at Fidentia. He was confident, but hesitated to attack as his men were 'without arms'. A gentle breeze covered his men with flowers from a nearby meadow, making them look like they were covered in garlands. This greatly encouraged them, and they attacked and defeated their opponents, killing 18,000 of them.
Orosius gives no date or location, but reports that Lucullus was besieged by Quintius (perhaps Licius Quinctius, a known opponent of Sulla), broke out and destroyed Quintius's army, killing 10,000 of them.
This fighting took place in the middle part of the Po valley, some 120 miles to the west/ north-west of the nearest other known fighting in the area, a failed attempt by Carbo to attack Metellus Pius's camp at Faventia, towards the south-eastern corner of the Po valley. Placentia (modern Piacenza) and Fidentia (Fidenza) are 23 miles apart, with Placentia on the Po and Faventia to its south-east, on the border between the Po valley and the Appenines.
These sources could be referring to two battles, one at Fidenza fairly early in the campaign of 82 BC and a second later on at Placentia, or to a single battle. Fidentia is just near enough to Placentia to fit with Appian's description, but was also a significant enough location to be named in its own right.
If this was a single battle, then we could reconstruct it as beginning with Lucullus being besieged by Quintius at Fidentia, breaking out, and defeating him on the plains near the town. However it could just as easily be two battles, Fidenti first, followed by Placentia.