<Tr> <Th> Opponents </Th> <Td> Goths, Huns, Sassanid Persia, Vandals, Ostrogoths, Avars, Slavs, Muslim Caliphate, Bulgaria, Rus', Normans, Crusader states, Seljuks, Anatolian beyliks, Ottomans and others </Td> </Tr> <P> The Byzantine army or Eastern Roman army was the primary military body of the Byzantine armed forces, serving alongside the Byzantine navy . A direct continuation of the Roman army, the Eastern Roman army maintained a similar level of discipline, strategic prowess and organization . It was among the most effective armies of western Eurasia for much of the Middle Ages . Over time the cavalry arm became more prominent in the Byzantine army as the legion system disappeared in the early 7th century . Later reforms reflected some Germanic and Asian influences--rival forces frequently became sources of mercenary units e.g.; Huns, Cumans, Alans and (following the Battle of Manzikert) Turks, meeting the Empire's demand for light cavalry mercenaries . Since much of the Byzantine military focused on the strategy and skill of generals utilizing militia troops, heavy infantry were recruited from Frankish and later Varangian mercenaries . </P> <P> From the seventh to the 12th centuries, the Byzantine army was among the most powerful and effective military forces in the world--neither Middle Ages Europe nor (following its early successes) the fracturing Caliphate could match the strategies and the efficiency of the Byzantine army . Restricted to a largely defensive role in the 7th to mid-9th centuries, the Byzantines developed the theme - system to counter the more powerful Caliphate . From the mid-9th century, however, they gradually went on the offensive, culminating in the great conquests of the 10th century under a series of soldier - emperors such as Nikephoros II Phokas, John Tzimiskes and Basil II . The army they led was less reliant on the militia of the themes; it was by now a largely professional force, with a strong and well - drilled infantry at its core and augmented by a revived heavy cavalry arm . With one of the most powerful economies in the world at the time, the Empire had the resources to put to the field a powerful host when needed, in order to reclaim its long - lost territories . </P> <P> After the collapse of the theme - system in the 11th century, the Byzantines grew increasingly reliant on professional Tagmata troops, including ever - increasing numbers of foreign mercenaries . The Komnenian emperors made great efforts to re-establish a native army, instituting the pronoia system of land grants in exchange for military service . Nevertheless, mercenaries remained a staple feature of late Byzantine armies since the loss of Asia Minor reduced the Empire's recruiting - ground, while the abuse of the pronoia grants led to a progressive feudalism in the Empire . The Komnenian successes were undone by the subsequent Angeloi dynasty, leading to the dissolution of the Empire at the hands of the Fourth Crusade in 1204 . </P>

What was the power structure of the byzantine empire