Roman Armies.

[Nathan Ross]

I was wondering why the Roman Military never expanded the use of Cavalry and Archers in their armies

The Romans always used cavalry to some extent - most recorded Roman armies are about 10% cavalry, which was normal for the period; they were often used on the flanks, where they could prove most effective. The detailed battle plan by Flavius Arrianus (early 2nd century) has the cavalry positioned behind the infantry front line, and attacking through lanes once the infantry have broken the impetus of the enemy charge - the enemy in this case being the mounted Alani.

Archers were used too; like the cavalry, these were auxiliary 'specialist' troops drawn from conquered peoples. Syrians in particular seem to have contributed large numbers of auxiliary archers during the principiate.

Most of the enemies of Rome during the earlier empire also fought largely on foot, and the classic legionary sword and javelin tactics were very effective against them. Against largely mounted opponents - like the Alani faced by Arrianus, or the Parthians on the eastern frontier - the Roman tactics would evolve. But the legionary infantry were always the mainstay of the force, and the real battle-winner.

From the 2nd century onwards, though, we see a more diverse array of cavalry types in Roman armies, from very light North African horsemen and Syrian horse archers to heavily armoured cataphractarii and, later still, clibanarii. But the expansion in the overall numbers of cavalry and archers in Roman armies does not seem to have happened until much later (later 4th or 5th century, perhaps), by which time far more of Rome's enemies were themselves mounted.