Gothic War (535–554) in the context of "Medieval Bosnia"

In modern historiography, the Western Roman Empire were the Roman Empire's western provinces, collectively, during any period in which they were administered separately from the eastern provinces by a separate, independent imperial court. Particularly during the period from AD 395 to 476, there were separate, coequal courts dividing the governance of the empire into the Western provinces and the Eastern provinces with a distinct imperial succession in the separate courts. The terms Western Roman Empire and Eastern Roman Empire were coined in modern times to describe political entities that were de facto independent; contemporary Romans did not consider the Empire to have been split into two empires but viewed it as a single polity governed by two imperial courts for administrative expediency. The Western Empire collapsed in 476, and the Western imperial court in Ravenna disappeared by 554, at the end of Justinian's Gothic War.

Though there were periods with more than one emperor ruling jointly before, the view that it was impossible for a single emperor to govern the entire Empire was institutionalized by emperor Diocletian following the disastrous civil wars and disintegrations of the Crisis of the Third Century. He introduced the system of the Tetrarchy in 286, with two senior emperors titled Augustus, one in the East and one in the West, each with an appointed subordinate and heir titled Caesar. Though the tetrarchic system would collapse in a matter of years, the East–West administrative division would endure in one form or another over the coming centuries. As such, the unofficial Western Roman Empire would exist intermittently in several periods between the 3rd and 5th centuries. Some emperors, such as Constantine I and Theodosius I, governed, if briefly, as the sole Augustus across the Roman Empire. On the death of Theodosius in 395, the empire was divided between his two infant sons, with Honorius as his successor in the West governing briefly from Mediolanum then from Ravenna, and Arcadius as his successor in the East governing from Constantinople.

Justinian I (Latin: Iustinianus, Ancient Greek: Ἰουστινιανός, romanized: Ioustinianós; 482 – 14 November 565), also known as Justinian the Great, was Roman emperor from 527 to 565.

His reign was marked by the ambitious but only partly realized renovatio imperii, or "restoration of the Empire". This ambition was expressed by the partial recovery of the territories of the defunct Western Roman Empire. His general, Belisarius, swiftly conquered the Vandal Kingdom in North Africa. Subsequently, Belisarius, Narses, and other generals conquered the Ostrogothic Kingdom, restoring Dalmatia, Sicily, Italy, and Rome to the empire after more than half a century of rule by the Ostrogoths. The praetorian prefect Liberius reclaimed the south of the Iberian Peninsula, establishing the province of Spania. These campaigns re-established Roman control over the western Mediterranean, increasing the Empire's annual revenue by over a million solidi. During his reign, Justinian also subdued the Tzani, a people on the east coast of the Black Sea that had never been under Roman rule before. He engaged the Sasanian Empire in the east during Kavad I's reign, and later again during Khosrow I's reign; this second conflict was partially initiated due to his ambitions in the west.

The Byzantine Empire under the Justinian dynasty began in 518 AD with the accession of Justin I. Under the Justinian dynasty, particularly the reign of Justinian I, the empire reached its greatest territorial extent since the fall of its Western counterpart, reincorporating North Africa, southern Illyria, southern Spain, and Italy into the empire. The Justinian dynasty ended in 602 with the deposition of Maurice and the accession of his successor, Phocas.

Flavius Belisarius (Latin pronunciation: [bɛ.lɪˈsaː.ri.ʊs]; Greek: Βελισάριος; c. 500 – March 565) was a military commander of the Byzantine Empire under Emperor Justinian I. Belisarius 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. He is considered one of the greatest military commanders of Byzantium and in history generally.

One of the defining features of Belisarius' career was his success despite varying levels of available resources. He is frequently cited as being among the "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.

Narses (also spelled Nerses; Latin pronunciation: [ˈnarseːs]; Armenian: Ներսէս; Greek: Ναρσής; c. 478–573) was a distinguished Roman general and statesman of Armenian heritage, renowned for his critical role in Emperor Justinian I’s military campaigns. Alongside the famed Belisarius, Narses was instrumental in the reconquest of Italy during the Gothic War (535–554), which sought to restore the western provinces of the Roman Empire.

Narses first gained significant recognition by suppressing the Nika riots in 532, using his diplomatic skills and wealth to help quell the revolt. His later campaigns in Italy were marked by decisive victories, most notably at the Battle of Taginae in 552, where he defeated the Ostrogothic king Totila, and at the Battle of Mons Lactarius in 553, effectively ending the Ostrogothic kingdom.

The Byzantine Papacy was a period of Byzantine domination of the Roman Papacy from 537 to 752, when popes required the approval of the Byzantine Emperor for their episcopal consecration, and many popes were chosen from the apocrisiarii (liaisons from the pope to the emperor) or the inhabitants of Byzantine-ruled Greece, Syria, or Sicily. Justinian I reconquered the Italian peninsula in the Gothic War (535–554) and appointed the next three popes, a practice that would be continued by his successors and later be delegated to the Exarchate of Ravenna.

With the exception of Martin I, no pope during this period questioned the authority of the Byzantine monarch to confirm the election of the bishop of Rome before consecration could occur; however, theological conflicts were common between pope and emperor in the areas such as monothelitism and iconoclasm.