Flavius Belisarius (Latin pronunciation: [fla.ui.s b.lsa.ri.s]; 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 Empire, also referred to as the Eastern Roman Empire or Byzantium, was the continuation of the Roman Empire in its eastern provinces during Late Antiquity and the Middle Ages, when its capital city was Constantinople. It survived the fragmentation and fall of the Western Roman Empire in the 5th century AD and continued to exist for an additional thousand years until the fall of Constantinople to the Ottoman Empire in 1453. During most of its existence, the empire remained the most powerful economic, cultural, and military force in Europe.

The terms "Byzantine Empire" and "Eastern Roman Empire" were coined after the end of the realm; its citizens continued to refer to their empire simply as the Roman Empire (Medieval Greek: Βασιλεία Ῥωμαίων, romanized: Basileía Rhōmaíōn) or Romania (Medieval Greek: Ῥωμανία), and to themselves as Romans (Medieval Greek: Ῥωμαῖοι, romanized: Rhōmaîoi) – a term which Greeks continued to use for themselves into Ottoman times. Although the Roman state continued and its traditions were maintained, modern historians distinguish Byzantium from its earlier incarnation because it was centred on Constantinople, oriented towards Greek rather than Latin culture, and characterised by Eastern Orthodox Christianity.

Several events from the 4th to 6th centuries mark the period of transition during which the Roman Empire's Greek East and Latin West diverged. Constantine I (r. 324–337) reorganised the empire, made Constantinople the new capital and legalised Christianity. Under Theodosius I (r. 379–395), Christianity became the state religion and other religious practices were proscribed. In the reign of Heraclius (r. 610–641), the Empire's military and administration were restructured and Greek was adopted for official use in place of Latin.

The borders of the empire fluctuated through several cycles of decline and recovery. During the reign of Justinian I (r. 527–565), the empire reached its greatest extent after reconquering much of the historically Roman western Mediterranean coast, including Africa, Italy and Rome, which it held for two more centuries. The Byzantine–Sasanian War of 602–628 exhausted the empire's resources, and during the Early Muslim conquests of the 7th century, it lost its richest provinces, Egypt and Syria, to the Rashidun Caliphate. It then lost Africa to the Umayyads in 698. During the Macedonian dynasty (10th–11th centuries), the empire expanded again and experienced the two-century long Macedonian Renaissance, which came to an end with the defeat by Seljuk Turks at the Battle of Manzikert in 1071. Civil wars and the ensuing Seljuk invasion led to the loss of most of Asia Minor. The empire recovered during the Komnenian restoration, and by the 12th century, Constantinople was the largest and wealthiest city in Europe. The empire was delivered a mortal blow during the Fourth Crusade, when Constantinople was sacked in 1204 and the territories that the empire formerly governed were divided into competing Byzantine Greek and Latin realms. Despite the eventual recovery of Constantinople in 1261, the Byzantine Empire remained only one of several small rival states in the area for the final two centuries of its existence. Its remaining territories were progressively annexed by the Ottomans in the Byzantine–Ottoman wars over the 14th and 15th centuries. The fall of Constantinople to the Ottoman Empire in 1453 ended the Byzantine Empire. Refugees fleeing the city after its capture would settle in Italy and other parts of Europe, helping to ignite the Renaissance. The Empire of Trebizond was conquered eight years later in the 1461 siege. The last of the successor states, the Principality of Theodoro, was conquered by the Ottomans in 1475.