The cataphract charge was very effective due to the disciplined riders and the large numbers of horses deployed. As early as the 1st century BC, especially during the expansionist campaigns of the Parthian and Sassanid dynasties, Eastern Iranian cataphracts employed by the Scythians, Sarmatians, Parthians, and Sassanids presented a grievous problem for the traditionally less mobile, infantry-dependent Roman Empire. Roman writers throughout imperial history made much of the terror of facing cataphracts, let alone receiving their charge. Parthian armies repeatedly clashed with the Roman legions in a series of wars, featuring the heavy usage of cataphracts. Although initially successful, the Romans soon developed ways to crush the charges of heavy horsemen, through use of terrain and maintained discipline.