Although the Romans won from time to time, no one considered them to be real warriors. They were just slaves, trained to walk in formation under shields. They were able to defeat only the West of Europe. In the area of Sormatia they were powerless against cavalry. Rome did not expand eastward. Expeditions in Asia also collapsed.
They were citizens, and if not citizens, their military service made them so. They were never slaves.
In fact, the cavalry element was made up of the wealthy land owners.
Ummm, what about the Eastern Roman Empire?
You don't know a damned thing about Rime.