By the late 4th century the pressures on the frontiers of the Roman Empire had transformed the nature of the army which defended it. With the western half of the empire in economic decline and chronic manpower shortages the army had proved incapable of defending the line of the frontier. Necessity forced the induction of large numbers of German federate allies and the transformation of the army into a mobile "fire-brigade" which could move from crisis-point to crisis-point, dealing with major barbarian incursions. This need for mobility was reinforced by Roman experiences against the Sassanid Persians increasing the quantity of armoured cavalry within the army. This book details the rise of the army and its evolution, after the collapse of the west, into the army of the Byzantine Empire.