The battle near Mutina (218 BC) saw a Roman army attempt to rescue Romans who were besieged at Mutina, only to suffer heavy loses when attacked by Gallic forces while passing through a forest, eventually escaping to safely at a Roman settlement on the Po.
Over the previous few years the Romans had won a series of victories over the Cisalpine Gauls, starting with the battle of Telamon (225 BC), which had seen Gallic force led by the Boii and Insubres crushed between two Roman armies (Telamon War). In the aftermath of this victory the Romans launched a series of invasions of Cisalpine Gaul, overrunning the Boii very quickly. The Insubres held out for longer, but submitted after the fall of Mediolanum (modern Milan) in 222 BC.
Over the next few years tensions began to rise between Rome and Carthage’s leaders in Spain, where Hannibal had come to power after the murder of his brother-in-law Hasdrubal in 221 BC. The Romans began to prepare for a probable war in Spain, and in 218 appointed the consul Publius Cornelius Scipio to lead the expedition to Spain. At the same time they decided to found two colonies on either side of the Po, Placentia and Cremona. These were on the southern boundaries of Insubres territory and were clearly intended to both secure Roman control of the area and provide bases for any further Roman campaigns in northern Italy. Each city was given 6,000 colonists, who were ordered to be at their new colonies within 30 days.
Unsurprisingly this alarmed the Gauls, who had acknowledged Roman supremacy, but still saw themselves as independent. By now the Boii and Insubres had recovered somewhat from their earlier defeats, and they were also encouraged by the prospect that Rome would soon be distracted by a fresh war with Carthage. The Boii rose first, followed by the Insubres. The two tribes then overran the lands allocated to the new colonies and the settlers fled east to Mutina (Modena), a slightly older Roman colony. Although some work had been done to fortify the new colonies it was clearly not complete, but Mutina must have been fortified, as the Romans were able to hold out.
When the siege began three senior Romans were trapped in the city, two former praetors and the former consul Caius Lutatius, who were there to organise the partition of Boii lands. These three men attempted to open negotiations with the Gauls, but were instead taken captive. The Gauls hoped that they could be exchanged for the Gauls who had been held hostage in Rome after the end of the last war.
At this point the Praetor Lucius Manlius was fairly nearly to Mutina at the head of a legion. He decided to march to Mutina to raise the siege, but instead he walked into an ambush. His route had to take him through a forest, and the Boii had time to prepare a series of hidden positions along his route. We have two accounts of this fighting, one from Polybius and one from Livy, which largely agree with each other.
According to Polybius when the Romans entered the forest they were attacked from all sides, and suffered heavy losses in the initial fighting. The survivors fled, but when they reached some higher ground were able to regain some shape, and were able to retreat to a place named Vicus Tannetis, where they were then besieged by the Gauls.
Livy also has Lucius Manlius marching through the forest and being attacked by the Gauls, but gives some extra details. Manlius sent his army off in separate columns and without sending scouts out, so they were vulnerable to attack. He was able to reach some open ground after losing about 500 men, and entrenched there. The Gauls refused to attack the Roman camp, and eventually Manlius was forced to move on. Things went well while his men were on open ground, but the Gauls attacked again when they had to pass though another forest. This time 700 men and six standards were lost, and the fighting only ended when the Romans emerged from the forest. They reached safely at Tannetum near the Po, where they entrenched themselves. They were joined by Brixian Gauls, who were Roman allies, and were able to hold off the Gauls.
Vicus Tannetis or Tannetum was a Roman settlement just to the east of modern Parma, about half way between Mutina and the new colonies at Placentia and Cremona, so the Romans had clearly been forced some way off their intended route by the Gallic attacks.
When news of this defeat reached Rome, another Praetor, C. Atilius Serranus, was given command of the legions that had been raised for Scipio’s campaign in Spain (one Roman legion and 5,000 allied troops according to Livy), and ordered to march to the relief of the besieged legion. Scipio was ordered to raise fresh troops from Rome’s allies, delaying his departure for Spain. The Gauls were unwilling to risk an attack on Atilius’s army, so he was able to rescue Manlius and his army. However he wasn’t able to rescue the three Romans captured at Mutina, as they were later offered to Hannibal.
At this point Livy and Polybius both shift their attention to Hannibal and his march from Spain to Italy, and we get no more details of Gallic affairs until after his dramatic crossing of the Alps and arrival in northern Italy (Second Punic War). Once Hannibal was clearly established and had achieved some early successes the Gauls joined him, including the Boii and Insubres.