Flavius Belisarius (Greek: ; c.500 565) was a military commander of the Byzantine Empire under the emperor Justinian I. He was instrumental in the reconquest of much of the Mediterranean territory belonging to the former Western Roman Empire, which had been lost less than a century prior.

One of the defining features of Belisarius' career was his success despite varying levels of available resources. His name is frequently given as one of the so-called "Last of the Romans".

He conquered the Vandal Kingdom of North Africa in the Vandalic War in nine months and conquered much of Italy during the Gothic War. He also defeated the Vandal armies in the battle of Ad Decimum and played an important role at Tricamarum, compelling the Vandal king, Gelimer, to surrender. During the Gothic War, despite being significantly outnumbered, he and his troops recaptured the city of Rome and then held out against great odds during the siege of Rome.

He also won an important battle against the Persians at Dara but was defeated at Callinicum. He successfully repulsed a Hunnish incursion at Melantias. He was also known for military deception; he repulsed a Persian invasion by deceiving their commander and lifted the siege of Ariminum without a fight.

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 itself on the legacy of the late Hellenistic armies, 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 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.

From the 7th 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.

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.

The Emperors of Nicaea managed to form a small but effective force using the same structure of light and heavily armed troops, both natives and foreigners. It proved effective in defending what remained of Byzantine Anatolia and reclaiming much of the Balkans and even Constantinople itself in 1261. Another period of neglect of the military followed in the reign of Andronikos II Palaiologos, which allowed Anatolia to fall prey to an emerging power, the Ottoman emirate. Successive civil wars in the 14th century further sapped the Empire's strength and destroyed any remaining chance of recovery, while the weakening of central authority and the devolution of power to provincial leaders meant that the Byzantine army was now composed of a collection of militias, personal entourages and mercenary detachments.