“You would not hesitate to call them the most terrible of all warriors,
because they fight from a distance with missiles [arrows] having sharp
bone, instead of their usual [metal] points, joined to the shafts with wonderful
skill; then they gallop over the intervening spaces and fight hand
to hand with swords, regardless of their own lives; and while the enemy
are guarding against wounds from the sharp [sword] points, they throw
strips of cloth plaited into nooses over their opponents and so entangle
them that they fetter their limbs and take from them the power of riding or
walking.”–Ammianus Marcellinus on the Huns
The Huns were first mentioned by Tacticus in 91 C.E (A.D.) as a steppe tribe living around the Caspian Sea. To the Romans of that period, the Huns were just one more horse barbarian tribe, and not of much concern. This would change in 376 C.E., when the Visigoths were drive into Roman territory by the migration westward of the Huns.
The Huns were one of the many steppe nomadic tribes who used the paradigm of the horse archer to define their warriors. This type of warrior was not new to the Romans of Late Antiquity. The horse archers of the ancient Scythian had been known since the early Classic time to Hellenistic writers. But the Huns were more dangerous than any of the past tribes.
The Huns trained their youth to ride horses from the age of 4 or 5, just as all the steppe nomads for millennia had done. So the horsemanship of the Huns was no surprised to the Romans, but the composite reflex bow of the Huns was a vast improvement over past composite bows the Romans knew. This bow combined with all the archery and horsemanship skills of the Huns created a warrior combination that the Romans and all the Germanic tribes had great difficulty facing.
The Hun could use the standard horse archer tactics of firing and retreating against less mobile forces at even greater ranges than the Romans had experienced in the past. There were few missilery troops the Romans had access to in this period that could match the range of the Hunnish bow. The classic counter tactic to the horse archer of using ground archers was not an option.
Other tactical attempts by the Romans to counter the Huns meet with only mixed results. In the end the Romans found it necessary to develop horse archers of their own, which also combined the capacities of heavy cavalry, to create mounted troops of a formidable nature. But these Roman cataphracts would prove expensive to train and maintain. Such troops had to be trained in effective horsemanship and archery, two skills the Huns had developed since childhood.
The self destruction of the Hun Empire in 469 C.E would spare the Eastern Roman Empire, but the Eastern Roman found their new elite troops highly effective against the Germanic tribes of Western Europe, and the Parthian eastern neighbors.
The cataphract had started become the main Roman unit in the 4th century, but the addition of the composite reflex bow was an Eastern Roman innovation to counter the Huns. These heavy cavalry, mixed with the comitatense heavy infantry would be main stands of the imperial armies as the Eastern Roman Empire became over time the Byzantine Empire.
This would be the standard till the Seljuk Turks defeated the Eastern Romans at the Battle of Manzikert in 1071, where much of the Byzantine army was destroyed. The Eastern Empire found it a costly defeat, and never fully rebuilt their army to such professional elitism again. Yet the root of these 600 years of effective cavalry combination of horse archer and heavy knight lie in the Roman answer to the legality of the Hunnish warriors.