At the close of the 4th century 298 BC, after only 6 years of peace, the Romans again found themselves at odd with the Samnites. Old wounds were slow to heal and the Samnites decided that intervention was still necessary to stop the Roman spread of power. The Third Samnite War became the last desperate attempt of the Samnites to remain independent and were able to convince Rome's old enemies to join against them once again. The Etruscans, the Umbrians, the Gauls, and other regional tribes joined in arms with Samnium to check Rome's ever increasing regional authority.
The war began again near Neapolis where possession of the plains of Campania had been a constant source of conflict. The threat to Rome was very real, facing newly coordinated enemies from all sides, the Samnites to the south and east, and the Etruscans and Gauls to the north. Until now, Rome had mostly been able to stand against single enemies at a time, but unification of these old foes made the challenge more difficult. This coalition of states not only created challenges, but proved the desperation of these tribes to reverse Roman expansion.
Early in the conflict, Rome shattered a Samnite army in the south, allowing attention to be focused on the Etruscans and Gauls in the north. The pivotal battle for Italy took place in 295 at Sentinum in Umbria, where, according to various sources, more combatants were engaged in a single encounter than at any time previous in Italy. Chariot driving Celtic Gauls forced the Romans back initially, but Roman discipline soon commanded the field, crushing the coalition against them. The Samnites, despite their less than favorable results, held out for 5 more years, finally capitulating in 290 BC.
Rome granted Samnium a favorably treaty forcing them into alliance and ending their unified resistance for the remainder of Rome's history. The Campanian cities both Italian and Greek, Capua and almost all others, were now the undisputed allies of Rome, with varying degrees of independence. The Sabines ceded territory as well, and several Roman colonies (13 known in total) were settled in Campania as well as on the eastern outskirts of Samnium.
The years surrounding the Samnite Wars were not only one of military prowess for Rome, but of great public works, as well. In 329 BC, the Circus Maximus got one of many face-lifts throughout its history, gaining permanent horse-stalls and starting gates. The first Roman road, the Via Appia was constructed from Rome to Capua in 312 BC and the first aqueduct, the Aqua Appia was also established at the same time. These magnificent structures not only were of great benefit to Rome and her people, but proved the flourishing disposition of the state even during time of war and expansion. At the end of the Samnite Wars, Rome held perhaps as many as 150,000 people making it one of the largest cities in the Mediterranean region. As many as 1,000,000 people claimed citizenship to Rome, and vastly larger numbers were obligated through Latin rights and allied status.
With the defeat of Samnium, the last major Italian threat, Rome was the master of nearly the entire Italian peninsula, save for the Gauls occupying the Po valley in the north and the Greek holdout cities like Tarentum in the far south. This growing power soon gained the attention of regional powers in Greece and later, the masters of the Mediterranean, the Carthaginians.