The Ostrogoths (Latin: Ostrogothi or Austrogothi) were a branch of the Goths (the other branch being the Visigoths), a Germanic tribe who developed a vast empire north of the Black Sea in the 3rd century AD and, in the late 5th century, under Theodoric the Great, established a Kingdom in Italy.

Invading southward from the Baltic Sea, the Ostrogoths, at the time known as the Greuthungi, built up a huge empire stretching from the Dniester to the Volga river and from the Black Sea to the Baltic shores. The Ostrogoths were probably literate in the 3rd century, and their trade with the Romans was highly developed. Their Danubian kingdom reached its zenith under King Ermanaric, who is said to have committed suicide at an old age when the Huns attacked his people and subjugated them about 370.

After their subjugation by the Huns, little is heard of the Ostrogoths for about 80 years, after which they reappear in Pannonia on the middle Danube River as federates of the Romans. However, a pocket remained behind in the Crimea when the bulk of them moved to central Europe, and these Crimean Ostrogoths existed until at least the 16th century. After the collapse of the Hun empire after the Battle of Nedao (453), the Ostrogoths under Theodoric the Great first moved to Moesia (c. 475–488) and later conquered the Italian Kingdom of the German warrior Odoacer. Theodoric became King of the Ostrogothic Kingdom in 493 and died in 526. A period of instability then ensued, tempting the Eastern Roman Emperor Justinian to declare war on the Ostrogoths in 535 in an effort to restore the former western provinces of the Roman Empire. Initially, the Byzantines were successful, but under the leadership of Totila, the Goths reconquered most of the lost territory until Totila's death at the Battle of Taginae. The war lasted for almost 20 years and caused enormous damage and depopulation of Italy. The remaining Ostrogoths were absorbed into the Lombards who established a Kingdom in Italy in 567 AD.

The Kingdom established by the Ostrogoths in Italy and neighbouring areas lasted from 493 to 553. In Italy the Ostrogoths replaced Odoacer, the de facto ruler of Italy who had deposed the last emperor of the Western Roman Empire in 476. The Gothic kingdom reached its zenith under the rule of its first king, Theodoric the Great. Most of the social institutions in the late Western Roman Empire were preserved during his rule. Starting in 535, the Byzantine Empire invaded Italy. The Ostrogothic ruler at that time, Witiges, could not defend successfully and was finally captured when the capital Ravenna fell. The Ostrogoths rallied around a new leader, Totila, and largely managed to reverse the conquest, but were eventually defeated. The last king of the Ostrogothic Kingdom was Teia. At its greatest extent the kingdom stretched from contemporary France in the west into modern Serbia in the southeast.

The Roman emperor was the ruler of the Roman State during the imperial period (starting at about 27 BC). The Romans had no single term for the office although at any given time, a given title was associated with the emperor. If a man was "proclaimed emperor" this normally meant he was proclaimed augustus, or (for generals) imperator (from which English emperor ultimately derives). Several other titles and offices were regularly accumulated by emperors, such as caesar, princeps senatus, consul and Pontifex Maximus. The power of emperors was generally based on the accumulation of powers from republican offices and the support of the army.

Roman emperors refused to be considered "kings", instead claiming to be leaders of a republic, however nominal. The first emperor, Augustus, resolutely refused recognition as a monarch. Although Augustus could claim that his power was authentically Republican, his successor, Tiberius, could not convincingly make the same claim. Nonetheless, the Republican institutional framework (senate, consuls, magistracies etc.) was preserved until the very end of the Western Empire.

Theodosius I (Latin: Flavius Theodosius Augustus; 11 January 347 – 17 January 395), also known as Theodosius the Great, was Roman Emperor from 379 to 395. Theodosius was the last emperor to rule over both the eastern and the western halves of the Roman Empire. During his reign, the Goths secured control of Illyricum after the Gothic War, establishing their homeland south of the Danube within the empire's borders. He also issued decrees that effectively made Christianity the official state religion of the Roman Empire.

He is recognized by the Eastern Orthodox Church as Saint Theodosius. He defeated the usurpers Magnus Maximus and Eugenius and fostered the destruction of some prominent pagan temples: the Serapeum in Alexandria, the Temple of Apollo in Delphi, and the Vestal Virgins in Rome. After his death, Theodosius' sons Arcadius and Honorius inherited the East and West halves respectively, and the Roman Empire was never again re-united.

Theodosius was born in Cauca or Italica, Hispania, to senior military officer Theodosius the Elder. He accompanied his father to Britannia to help quell the Great Conspiracy in 368. He was military commander (dux) of Moesia, a Roman province on the lower Danube, in 374. However, shortly thereafter, and at about the same time as the sudden disgrace and execution of his father, Theodosius retired to Spain. The reason for his retirement, and the relationship (if any) between it and his father's death is unclear. It is possible that he was dismissed from his command by the emperor Valentinian I after the loss of two of Theodosius' legions to the Sarmatians in late 374.