The twilight of the Roman Empire saw a revolution in the way war was waged. The drilled infantryman, who had been the mainstay of Mediterranean armies since the days of the Greek hoplite, was gradually replaced by the mounted warrior. This change did not take place overnight, and in the 3rd and 4th centuries the role of the cavalryman was primarily to support the infantry. However, by the time of the 6th century, the situation had been completely reversed. Late Roman Cavalryman gives a full account of the changing experience of the mounted soldiers who defended Rome's withering western empire.
Late Roman Cavalryman AD 236-565 by Simon MacDowall