Written by: Dave Galvin
Soldiers on horseback provide mobility and speed to an army commander. Apart from the fortunate wealthy few in Republican times who could afford to provide a horse, the army consisted of stocky landowners, men who marched to war, fought a summer campaign and then returned home. This system continued until Romes expansion brought it into conflict with different modes of warfare. Their large, highly disciplined but static legions now stood before far quicker squadrons of cavalry and even elephants. The war against Hannibal 218-202BC, and Crassus' defeat at Carrhae in 53BC against the Parthians for example.
Not wishing to be outdone, Rome's expansion, as piecemeal as it was proving to be prior to the change to Imperial times, brought it into contact with horse dominant societies. Tribes were cajoled, bribed or forced into service alongside the legions. Young men from over much of central Europe were seeing places that they had never dreamt of, fighting for a city they had never seen. Its pay and citizenship being the reward.
By the time of the Claudian invasion of Britain in 43AD and there afterwards, the use of horses to augment the legions was well developed. This is testified by the tombstones of Longinius in Colchester and Genialis in Cirencester. The Ala Prima Thracum is also noted alongside the II Augusta at Caerleon.
An approximate disposition of cavalry to legions would be in the order of 2:1. Two ala to every Legion. Under the Emperor Augustus for example, Hispania had 3 legions and 7 alae; Germania 10 legions and 16 alae; Dalmatia and Panonnia 6 legions and 7 alae. By the time of Claudius, Britannia had 4 legions with 9 alae (mostly drawn from Germania and Panonnia), Germania 8 legions and 10 alae, Hispania 2 legions and 6 alae.
Horses were sent to where they were needed and especially to where warfare was still being waged in newly won territories and to where operating wide scale in remote mountainous regions was proving difficult for the legions. Horsemen and light armoured auxiliaries provided a more flexible military tool. Their mobility sent them further from their bases and could as such scatter and disperse enemy forces hiding from the Roman legions and the inevitable set piece pitched battle.
