The First Samnite War (343-341 BC) was the first of three clashes between Rome and the Samnite hill tribes, and ended in a Roman victory that saw the Republic begin to expand into Campania.
The first war broke out as a result of a Samnite attempt to expand to the west. In 343 BC they attacked the Sidicini, a smaller tribe on their south-western border. The Sidicines called for help from Campania, their southern neighbours. Capua, the most important Campanian city state and leader of a northern Campania league, responded to the call, but their army was badly defeated on Sidicini territory. The Samnites then advanced south into Campania, defeated the Campanians for a second time outside Capua, and then besieged the city.
The Campanians sent envoys to Rome to ask for help. This presented the Romans with a problem. The Samnites had been their allies for the last decade, but the Romans couldn't afford to see them expand into the fertile coastal plains of Campania. According to Livy at first the Senate refused to turn on their allies. The Campanians responded by surrendering their league into Roman hands. This forced the Senates hands. Envoys were sent to the Samnite assembly asking them not to attack the new Roman territory. When the Samnites refused to agree to this request the Romans declared war.
Both of the consuls for 343 B.C. led armies against the Samnites. Marcus Valerius Corvus was sent into Campania, while Cornelius Arvina invaded Samnium.
Valerius was the first to clash with the Samnites. He advancd south into Campania, eventually reaching Mount Gaurus, west of Naples and some way to the south of Capua (Livy doesn't say if this move forced the Samnites to lift the siege of Capua). A hard fought battle followed (battle of Mount Gaurus), which lasted until night fall, by which time the Romans had the upper hand. The Samnites withdrew from the battle field, and the siege of Capua was definitely lifted.
Cornelius Arvina led his army just across the border into Samnium. As he advanced from Saticula he marched into a Samnite trap in a narrow valley. He was saved from disaster by P. Decius Mus, a military tribune, who led part of the army onto a high peak overlooking the valley. This caused enough confusion for the main army to escape the trap, and on the following day the Samnites suffered a second defeat.
Throughout the Samnite Wars the Samnites showed an impressive ability to form new armies (or suffered much smaller losses than recorded by the Romans). A new Samnite army was formed, and advanced into Campania. Valerius marched back towards Capua from his camp, presumably still at Mount Gaurus, and won his second victory of the war at Suessula.
This victory effectively ended the war, although peace was not restored until 341. Part of the Roman army mutinied in 342, preventing the Republic from taking any offensive action, and by 341 BC it was clear that Latium was about to revolt. The consul L. Aemilius Mamercus led an army into Samnium, where he was met by Samnite peace envoys. They asked for an end to the war and the right to continue their attack on the Sidicines. The Romans agreed to these terms, in return for an indemnity equivalent to one years pay for the army.
This change of attitude can probably be explained by the outbreak of the Latin War. It drove the Campanians and Sidicini into the Latin camp, effectively reversing the alliances of the Samnite War (presumably the Romans believed that the Samnites were a more dangerous threat than the Campanians), and meant that part of the Latin War would be fought in Campania.