- The Appian Way gave the Romans a great advantage when they needed reinforcements during their many battles.

- Before Roman strength increased further, a marauding Gallic tribe swept down from the Po River valley, raided Etruria, and descended upon Rome. The Romans were defeated in the battle of the Allia River in 390 BC, and the Gauls captured and sacked the city; they departed only after they had received ransom in gold.

- After this the Romans greatly feared and respected the strength of the Gauls.

- Much of the fighting during this time consisted of three wars against the Samnites, Oscan-speaking tribes of the central and southern Apennines.

- The Samnite tribes united, no doubt, to resist Rome's expansion.

- Both the rugged terrain and the powerful Samnite soldiers proved to be difficult challenges, which forced Rome to adopt innovative military methods that later became important for conquering the Mediterranean.

- Once invited in, Rome usually absorbed the allied state after defeating iany of their enemies.

- Campania was a major addition to Rome's strength and manpower.

- The absorption of Campania provoked the Latins to take up arms against Rome to maintain their independence.

- Since the Gallic sack of Rome in 390 BC, the city had become increasingly dominant within the Latin League.

- In 381 BC Tusculum was absorbed by being given Roman citizenship.

- In 358 BC Rome created two more tribes from territory captured along the Volscian coast.

- The Latin War (340-338 BC) was quickly decided in Rome's favour. Virtually all of Latium was given Roman citizenship and became Roman territory, but the towns retained their local governments.

- The large states of Praeneste and Tibur maintained somewhat of an independence by becoming Rome's military allies.

- The Latin League was abolished; but the legal rights that the Latins had enjoyed among themselves were retained by Rome as a legal status, the Latin right (ius Latii), and used for centuries as an intermediate step between non-Roman status and full Roman citizenship.

- Rome was now the master of central Italy and spent the next decade organizing and pushing forward its frontier through conquest and colonization.

- The Romans soon confronted the Samnites of the middle Liris (modern Liri) River valley, sparking the Second, or Great, Samnite War (326-304 BC). During the first half of the war Rome suffered serious defeats, but the second half saw Rome's recovery, reorganization, and ultimate victory.

- In 321 BC a Roman army was trapped in a narrow canyon near the Caudine Forks and compelled to surrender, and Rome was forced to sign a five-year treaty.

- In 315 BC, after the resumption of hostilities, Rome suffered a crushing defeat at Lautulae. Ancient sources state that Rome initially borrowed hoplite tactics from the Etruscans (used during the 6th or 5th centuries BC) but later adopted the manipular system of the Samnites, probably as a result of Samnite success at this time.

- The manipular formation resembled a checkerboard pattern, in which solid squares of soldiers were separated by empty square spaces. It was far more flexible than the solidly massed hoplite formation, allowing the army to maneuver better on rugged terrain. The system was retained throughout the republic and into the empire.

- During these same years Rome organized a simple navy, constructed its first military roads (construction of the Via Appia was begun in 312 BC and of the Via Valeria in 306), and increased the size of its annual military levy.

- During the period 334-295 BC, Rome founded 13 colonies against the Samnites and created six new tribes in annexed territory. During the last years of the war, the Romans also extended their power into northern Etruria and Umbria. Several successful campaigns forced the cities in these areas to become Rome's allies.

- The Great Samnite War finally ended in Rome's victory. During the final phase of this war, Rome, on another front, concluded its third treaty with Carthage (306 BC), in which the Carthaginians acknowledged all of Italy as Rome's sphere of influence.