The Germanic tribes along the Rhine provide the armies of the Res Publica with an additional source for an effective battlefield cavalry.
While the majority of Germanics mainly rely on their infantry some tribes are famous for the horsemen and good quality cavalry. Their manoeuvres are surprisingly well executed and the horsemen are taught to fight as a unit even without roman training. The Equites Germanorum in roman service are an excellent light to medium cavalry force, suited to fight in skirmishes as well as shock actions. As most of the other tribal warrior these men are armed with several frame, long spears with a iron tip that can be thrown as well, and a wooden shield but other than ordinary Germanic cavalry the roman auxiliaries are usually also equipped with chain mail shirts and often ride bigger horses of Mediterranean breeds. Thus they can fight at close quarters for an considerably longer time and with much higher chances to succeed.
Historically, the speciality of the Germanic cavalry was to fight together with an equal number of swift picked light infantrymen, that could either support them directly in melee or form a compact body for the riders to retreat. A practice that was continued in roman service at least into Augustan times when the auxilia, and especially the cavalry, became more standardized and the tribes that permanently stayed under roman rule continuously more Romanized. Germanic cavalry commonly appeared in roman service since their frontier had reached the Rhine during Julius Caesar's campaigns in Gaul. While Germanic mounts were generally seen as too small and inadequately trained for cavalry service by the Romans, Germanic horsemen were often reequipped with Iberian and other Mediterranean breeds, the warriors of Germania had an excellent reputation and were especially famed for their loyalty towards their leader. Many of them served as bodyguards of late republican warlords during the decades of civil war and the early emperors were protected by an approximately cohort strong private Germanic guard with a strong mounted component. During its existence the soldiers of the corporis custodes germani were mostly recruited from the tribes of the Batavians and Ubians, and thus were popularly known as batavi in Roma. After the end of the Julio-Claudian dynasty in 68 AD the new emperor Galba disbanded the unit.
Allied and subdued states and tribes always had to supply the roman army with troops. Almost at all times at least 50% of Roma’s soldiers were non-citizens. During the first centuries of the republic the old alae of the Italic socii were organized and equipped in a similar way to the roman legions, but around the beginning of the 1st century BC the situation changed. After the social war almost all free people of Italia received roman citizenship and could now be recruited into the regular legions. In the decades after the “marian reforms” the roman light infantry disappeared. The ordo equester, since decades unable to provide a sufficient number of cavalry for the many wars the late republic had to fight, had split off in two main groups. A mainly political elite that filled out the numerous officer and administrative posts the ever growing Res Publica had to offer, and a pure economical elite, the large majority of the Roman and Italic equestrians.
The various different peoples now ruled by the republic were a much more heterogeneous group than the former Italic socii. Depending on their relative military strengths, they had to supply the roman army with the various troop types needed, besides heavy infantry, in order to be competitive on the battlefield.
Most of these troops were levied in the surrounding areas under roman control prior to campaign and only large scale or intesively prepared wars demanded additional forces and specialised units from distant areas of the Roman world. Many of the auxiliaries fought under their own chieftains and officers, while others, especially those recruited from regular provinces, were commanded by roman officers and even organized in roman manner. Usually the auxiliary units were disbanded and sent home after the end of a conflict. However, in the continuous wars during the last decades of the republic many served so long alongside the legions and fought for their generals that they were largely Romanized in the end.