Roman army of the late Republic

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The Roman army of the late Republic refers to the armed forces deployed by the late Roman Republic, from the beginning of the first century BC until the establishment of the Imperial Roman army by Augustus in 30 BC.

Shaped by major social, political, and economic change, the late Republic saw the transition from the Roman army of the mid-Republic, which was a temporary levy based solely on the conscription of Roman citizens, to the Imperial Roman army of the Principate, which was a standing, professional army based on the recruitment of volunteers.^[1]

Continuous expansion, wars, conflicts, and the acquisition of a growing, overseas territory led to an increasing degree of professionalism within the army.^[2] The late-Republic saw much of its action taking place within the Roman borders and between Roman commanders as they vied for control of the Republic. There was a significant intertwining of military and politics in the acquisition and maintenance of power. After the Social War, and following the establishment of the First Triumvirate by Julius Caesar, Licinius Crassus, and Pompeius Magnus, there grew an emphasis on the expansion of a united Republic toward regions such as Britain and Parthia. The effort to quell the invasions and revolts of non-Romans persisted throughout the period, from Marius’ battles with the wandering Germans in Italy to Caesar's campaign in Gaul.

After the completion of the Social War in 88 BC, Roman citizenship was granted to all its Italian allies (the socii) south of the Po River. The alae were abolished, and the socii were from now on recruited directly into uniformly organized and equipped legions.^[3] The non-Italian allies that had long fought for Rome (e.g., Gallic and Numidian cavalry) continued to serve alongside the legions but remained irregular units under their own leaders.

For reasons that remain uncertain to this day, the structure of the Roman army changed dramatically during the late Republic. The maniple, which had been the standard unit throughout the mid-Republic, was replaced by the cohort as the new standard tactical unit of the legions, while the Roman citizen cavalry (equites) and light infantry (velites) disappeared from the battlefield. Traditionally, many of these changes have been attributed to the reforms of Gaius Marius (see Marian reforms), but some scholars argue that they may have happened far more gradually.^[4]