The battle of Veseris (or Vesuvius) of 340 BC was the first major battle of the Latin War of 340-338 BC and was a Roman victory made famous by the execution of the young Manlius Torquatus by his father, the consul Manlius Torquatus and the self-sacrifice of the consul Decius Mus.
At the start of the Latin War the combined Latin and Campanian armies were based around Capua. The Romans responded by sending both consuls into Campania, to operate together. This reflects the serious nature of the threat to Rome, as normally the two consuls operated separately.
The Latin revolt posed a direct threat to Roman power. As a stroke it halved the strength of the Roman army by removing their normal allies. Worse, the Latins fought in the same way as the Romans, with the same equipment and organisation. The battle of Veseris would resemble to battles of later Roman civil wars, with legions on both sides (both sides also had allies with them - Samnites for the Romans and Campanians for the Latins, but their role in the battle, if any, is not clear).
After a period of manoeuvring that isn't detailed by Livy the two armies reached the vicinity of Mount Vesuvius, close to the Veseris river. At some point during this period the first famous incident of the battle took place. The consuls were aware that their men and their Latin opponents had often served alongside each other, and were worried about what would happen if the two sides were allowed to mingle, they ordered that no one was to leave his post to fight the enemy without orders.
After this order was issued a patrol led by T. Manlius Torquatus, the consul's son, ran into a Latin force led by Geminus Maecius, a well known Tusculan warrior. He challenged Manlius to a single combat, and despite his father's orders Manlius accepted the challenge. He won the duel, but on his return to the Roman camp was arrested, and to maintain discipline was beheaded. At the time this made Torquatus the consul desperately unpopular, although he later became a model of Roman virtue.
The second famous incident can be traced back to Decius Mus's original rise to fame, at the battle of Saticula (343 B.C.), during the First Samnite War. Here Mus had saved a Roman army from a Samnite trap, and according to Livy at one point had inspired his men by recounting a dream in which he would die a glorious death in battle. Now, with when the consuls were camped outside Capua, they both had a dream in which a dreadful presence told them that one of the consuls and the enemy army had been dedicated to the gods of the under world. The omens continued to come.
As the two armies prepared for battle the Romans consulted their soothsayers. They announced that the future looked bleak for Decius Mus, but good for his fellow consul and for the army. These omens all pointed to the devotio, a ritual in which one of the consuls and the enemy army would indeed by dedicated to the gods of the under world. That consul would then have to throw himself into the enemy lines and seek to be killed. The two consuls now agreed that the commander of whichever wing of the army buckled first would carry out the devotio.
When the battle began Manlius commanded the Roman right and Decius the left. As the Romans had expected the two lines were very equally balanced, and eventually the hastati on the Roman left were forced to retreat behind the principes. Decius Mus took this as a sign that it was time to perform the devotio. After carrying out the required ceremonies he charged into the Latin lines, and was promptly killed.
This left Manlius Torquatus in command of the entire Roman army. Despite his colleague's sacrifice the Latins were still pressing heavily on the Roman lines, and it was almost time for Torquatus to call on the triarii, the third and final line of the legions. Instead he called the accensi forward. These were the lightest troops in the legion, normally armed with slings, but on this occasion the Latins apparently mistook them for the triarii, and threw their own triarii into the battle. After these Latin reserves had tired themselves out, Manlius finally unleashed his own triarii. The Latin front line was destroyed, and the rest of the army defeated almost without a fight. According to Livy one a quarter of the Latin force survived.
The role of the Samnites and Campanians in the battle is unclear, although it seems likely that they fought each other. By the end of the battle the Latin and Campanian camp had been captured, and many Campanians killed there, and the Samnites were threatening the Latin flanks.
This defeat seems to have knocked the Campanians out of the war, but the Latins retreated back north towards their homeland, receiving reinforcements as they went. They were followed by the Roman army, now under control of the sole remaining consul, and suffered a second defeat at Trifanum.