The Latin War of 340-338 was a major step in the road that led to Roman control of the Italian peninsula, and that saw a major change in the relationship between the Roman republic and her former Latin allies.
The Latin War of 498-493 had been ended by a treaty in which Rome and the Latin League were treated as equals. There was to be a perpetual peace between the two sides, neither was to assist or give free passage to the enemies of the other, and they were to split the spoils of any successful joint campaign equally. Over time the Latin League became increasingly dissatisfied with the way the Romans were interpreting this treaty, and came to feel that they were being treated as subjects rather than allies. Even so, for most of the century and a half between the two Latin Wars the Latin League supplied half of the strength of most Roman armies, and Roman and Latin armies became increasingly similar.
The Latin War overlapped with the First Samnite War. In 343 the Romans had won a number of victories, but in 342 they had been distracted by an army mutiny. This may have encouraged the Latin League to operate independently, for in the same year they attacked the Paelignians. It also encouraged the Volscians, who controlled a large area to the east of Latium. In 341 one Volsci force, from Privernates, attacked the Roman colonies of Setia and Norba, while another army, led by the Volsci of Antium, mustered at Satricum.
The Romans were forced to split their armies. The consul C. Plautius was sent against the Volsci, defeating both of their armies, although he was unable to seize Antium. His colleague Aemilius Mamercus led the army into Samnium, where he was greeted by peace envoys. The Romans agreed to end the war in return for one years pay for their armies, while the Samnites were given a free hand against the Sidicines. It had been a Samnite attack on the Sidicines that had led to the First Samnite War, although only after the Campanians had become involved. To save themselves from the Samnites they had surrendered themselves to the Romans, but the events of 341 soon made them regret that decision.
The Samnites sent an army against the Sidicines, who now asked for help from Rome. The Romans turned them down, and so the Sidicines turned to the Latin League. The Latins agreed to help, and a combined Latin, Sidicini and Campanian army invaded Samnium. The Samnites turned to Rome for help, but got a vague answer. The Senate was willing to order the Campanians to stop fighting, but there was nothing in their treaty with the Latin League to prevent them from making war on whoever they liked. This answer drove the Campanians further into the planned revolt.
By now it was becoming clear in Rome that the Latin League was preparing for war, and so the Senate decided to ask ten chiefs of the league to come to Rome to present their demands. According to Livy the Latin League wanted to be treated as equals of Rome, with half of the senators and one of the consuls for each year to come from Latium. These demands (which are probably too similar to those of the Italian rebels of the Social War of 91 BC to be accurate) were refused at a dramatic meeting with the senate, and war was declared.
The Roman army of 340 was commanded by the consuls Decius Mus and Manlius Torquatus, each with two legions, but of course without their usual Latin allies. The Roman army advanced along one of two possible routes into Campania. According to Livy they took a long route across the lands of the Marsi and Paeligni into Samnium before emerging into Campania. Dionysius of Halicarnassus gives a different, more direct route, along the route that later became the Via Appia. In either case the Romans eventually reached Capua, camping five miles away from the city, where the Latins and Campanians were based. Livy's route is sometimes discounted as being unrealistically complicated, but it would have had the benefit of being unexpected, and of bringing the Roman army into enemy territory from an unusual direction.
The next part of the campaign is unrecorded, but it ended with the two armies facing each other near to Mount Vesuvius, on the road to the River Veseris, twenty five miles to the south of Capua. Livy records the resulting battle of Veseris as a clash between Roman and Latin armies that were equipped, organised and fought in the same way. He largely ignores and role played by the Samnites or Campanians. The hard-fought battle was most memorable for the self-sacrifice of the consul Decius Mus, who performed the devotion, a ritual that dedicated both himself and the opposing army to the gods of the underworld, although it was actually the leadership of his fellow consul Manlius Torquatus that won the battle. The defeated Latins retreated north-west, where they received reinforcements, before suffering a second defeat at Trifanum.
This ended the fighting in Campania. The Latins and Campanians lost large areas of their territory, which became part of the ager Romanus and was distributed amongst the plebs. Of the Latin states only Lavinium was not punished, having chosen not to join the revolt.
Nearer to home the Romans still faced a threat from Antium, and the Antiates even raided Ostia, the port of Rome. T. Manlius was now in poor health, and so appointed L. Papirius Crassus as dictator, but he failed to make any real impact, despite spending several months camped in Antiate territory.
In 339 the war moved much closer to Rome. The Latin states decided to fight rather than accept the loss of their lands. The centre of the revolt was the small city of Pedum, just over fifteen miles to the east of Rome. The Latin cities of Tibur, Praeneste, Lanuvium and Velitrae, and the Volscian city of Antium all provided troops, and the Roman consul Tiberius Aemilius Mamercinus was unable to capture the city. His colleague Quintus Publilius Philo won a victory at the unknown Fenectane Plains, defeating an unspecified set of Latin cities, and according to Livy Mamercinus abandoned the siege of Pedum when his colleague was awarded a triumph.
After the failures of 339 the Romans made the capture of Pedum their main aim for 338. The Latins, aware that they were too weak to risk open battle, decided to focus on the defence of their cities, so when the Romans threatened Pedum the armies of Tibur and Praeneste marched to defend the city. A second Latin army, with contingents from Aricia, Lanuvium and Velitrae, moved south to join up with the Volscians from Antium.
The Romans also split their forces. The consul Gaius Maenius moved south, and defeated the allied army at the battle of Astura. Antium fell soon afterwards. His colleague Lucius Furius Camillus moved against Pedum, where he defeated the armies from Tibur and Praeneste. During the battle the defenders of Pedum made a sortie which went badly wrong, and Camillus was able to storm the city.
In the aftermath of these two victories the consuls moved through Latium, capturing every Latin city. They then returned to Rome to celebrate their triumphs and to help arrange a new settlement for Latium. The old Latin League disappeared, and most Latin cities lost the rights of intermarriage, free trade and common council. A few cities were singled out for special treatment. Pedum, Lanuvium, Nomentum and Aricia were given Roman citizenship. Tusculum retained its former status. A new body of Roman colonists were sent to Antium, although the existing citizens of the city were allowed to enrol in the new settlement.
Velitrae, Tibur and Praeneste were the most harshly treated. Tibur and Praeneste lost all of their lands. Velitrae had its walls destroyed and its senate deported to the west bank of the Tibur. New Roman colonists were brought in to fill the gap. The result was another major expansion of the ager Romanus, and in the number of Roman citizens.