Rome’s legions of citizens were formed under the control of the Consuls at the beginning of the Sixth Century BCE, between the end of the wars against the Latins and the Etruscans and the critical transition from monarchy to republic in 509 BCE. Within a century its reformed army, diplomatic successes and the founding of its first colonies enabled Rome to overcome the traumatic sacking of the city by the Celts in 390 BCE and to assert itself over the Italic coalitions (295-290 BCE).
The Romans managed to defeat their great Third Century BCE enemies such as King Pyrrhus and his war elephants (279 BCE), or the Carthaginians, with their ability to adapt to different war scenarios both on land and sea (as demonstrated by, among other technologies, their use of the rostrum and hooked bridges for boarding enemy vessels in naval battles during the First Punic War).
Rome’s success derived from a systematic policy of expansion pursued tirelessly over several generations by certain patrician families. One such were the powerful Scipios, who waged war against Carthage, eventually destroying it in 146 BCE at around the same time that Rome conquered Greece.
After defeating Cimbrian and Teutonic invaders, Gaius Marius undertook a reform of the army (107 BCE) which resulted in the definitive removal of patrimonial requisites for admission into military service, ushering in the professional figure of the legionnaire and the inclusion in the Roman legions of paid mercenaries.
The considerably larger armies now in place, coupled with new socio-economic scenarios brought about by Rome’s conquests in the east, inaugurated a period of bloodshed, clashes with the Italic socii who demanded Roman citizenship (91-87 BCE) and twenty years of civil war as opposing factions – under the command of great military leaders such as Sulla, Pompey the Great or Caesar – wrestled for control of the state (43-31 BCE). The great naval battle of Actium, in which the forces of Octavian prevailed over those of Mark Antony and Cleopatra of Egypt (31 BCE), marked the end of a prolongued process which culminated in political unity and territorial expansion for the Roman Republic.