After the end of the Latin War in the 330's BC, the Romans expanded into the territory of the Aurunci and Sidicini to the south of the Volsci. They also attempted to reassert control of Campania by moving south across the Liris River. In 328 the Romans, clearly looking for another fight with Samnium, established a colony at Fregellae on the Liris in and another at Cales, earlier in 334 BC.
The Samnites, of course, found this to be an unacceptable intrusion by Rome, but were too pre-occupied to respond immediately. They were involved in a conflict with the Greek colony of Tarentum and its ally, King Alexander of Epirus. At the end of this war, in 331 BC, the Samnites were free to deal with the reality of Roman expansion. The Romans had claimed that the Samnites were encouraging the people of Neapolis to expand into the territories of Campania and necessitated the creation of colonies in disputed areas. The Samnites, in response, sent troops to garrison Neapolis (modern Naples), and the elite class called to Rome for help. In 327 BC, a Roman army arrived and threw out the Samnite garrison, setting off the Second Samnite War.
By the beginning of this renewed war, the Samnites controlled approximately twice as much territory, though mostly mountainous and not as fertile, as the Romans. Initially, the war went clearly in the favor of Rome, even prompting Samnium to sue for peace in 321 BC. The Romans over-confidant, offered terms that were so lopsided that the Samnites rejected them, and the war continued. While seemingly in dire straits, the Samnites would learn to use their mountainous terrain to their advantage, and turn the tides.
Later in 321 BC, the two Consuls for that year advanced a Roman army deeper into Samnite territory. The territorial advantaged Samnites, at what would become the Battle of the Claudine Forks, soon trapped the Romans in a mountain pass. Finding themselves completely surrounded and faced with certain annihilation, the Romans capitulated and were forced to march out under a "yoke of spears". The Romans were forced to give up their spears and march under them, a sign of the ultimate battlefield humiliation. Some sources suggest that Six hundred equites had to be handed over as hostages and the Romans had to pledge a five-year treaty while also giving up her colonies at Fregellae and Cales. Later Roman historians, however, tried to claim that these terms were rejected, but its quite clear that operations against Samnium did cease until about 316 BC.
In this 5-year respite, the Romans took the opportunity to strengthen their military position. In 318 they absorbed two more regional tribes, the Oufentina at the south of Volsci territory, and the Falerna to the north of Capua. They also surrounded the Samnites with Roman allies by attacking and overtaking the Apulia and Lucania to the east and south of Samnium. Several more tribes were forced to take allied status with Rome, further increasing the pressure on the Samnites.
When military operations resumed in 316, however, Rome still found itself on the losing side of the conflict. They were defeated in several successive engagements including a crushing defeat at Lautulae in 315. Within a year, Campania was on the verge of rejecting Rome and joining the Samnites, so the Romans were forced to sue for peace again with some of the Samnite factions. The Samnites, however, kept the pressure on by encouraging the Etruscans of Etruria to join them. By 311, at the end of a forty-year treaty, the Etruscans joined the conflict, but just at the time the tide was beginning to turn.
Initially the Romans were continuously defeated by both of their enemies, but between 311 and 304, they won a series of victories against both the Etruscans and the Samnites. In 308 BC the Etruscans were forced to capitulate on severe terms and in 304 BC the Samnites followed suit. While not conquered, the Samnites were severely weakened, and Rome, despite the struggle, came to take considerable territory where many new colonies were established.
In addition to the gain of territory, some ancient sources suggest that the Romans adopted the Manipular military formation of the Samnites as a result of their early successes. It was far more flexible than the hoplite system of the Greeks and Etruscans that Rome had been using, and allowed great maneuverability on all sorts of terrain and conditions. The system was in use throughout the Republic and later evolved into the cohort formation that would later conquer Europe.
The Second Samnite War is a perfect example of Rome's long-range campaign tactics and how planning for the long term would nearly always pay off. As a result of this strategy, the construction of the Via Appia (by Censor Appius Claudius) was begun in 312 and the Via Valeria in 306 BC. The Via Appia, covered the 132 miles between Rome and Capua in Campania, and provided a fast moving highway for the early legions to advance against the Samnites. The first of many remarkable Roman engineering achievements, literally paved the way for the conquest of Southern Italy.
This final decade of the fourth century was the culmination of resistance to Roman domination by several neighbors. The Aequi and Hernici both revolted and joined the Samnites. Several other previously unmolested tribes, the Marsi, Marrucini, Paeligni, Frentani and Vestini, also joined Samnium against Rome. Their efforts were too late to stop the spread of Roman expansion and in 305 BC a Roman victory led the Paeligni and Hernici to surrender. In 304 the Aequi were defeated in the same year the Samnites sued for peace, and all the other tribes of Central Italy would make alliances with Rome within another 2 years. The Samnites were still a thorn in Rome's side, however, and conflict would be renewed within the decade.