Just as what many today label the Byzantine Empire was in reality and to contemporaries a continuation of the Roman Empire, so the Byzantine army was an outgrowth of the Late Roman structure, which largely survived until the mid–7th century. The official language of the army for centuries continued to be Latin but this would eventually give way to Greek as in the rest of the Empire, though Latin military terminology would still be used throughout its history. In the period after the Muslim conquests, which saw the loss of Syria and Egypt, the remnants of the provincial armies were withdrawn and settled in Asia Minor, initiating the thematic system. Despite this unprecedented disaster, the internal structures of the army remained much the same, and there is a remarkable continuity in tactics and doctrine between the 6th and 11th centuries. The Battle of Manzikert in 1071 and the subsequent Seljuk invasions, together with the arrival of the Crusades and the incursions of the Normans, would severely weaken the Byzantine state and its military, which increasingly had to rely on foreign mercenaries.

The Byzantine army was the primary military body of the Byzantine armed forces, serving alongside the Byzantine navy. A direct continuation of the Eastern Roman army, shaping and developing itself on the legacy of the late Hellenistic armies,^[1] it 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, such as the 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.