Gothic War (376–382) in the context of "Theodosius the Great"

The fall of the Western Roman Empire, also called the fall of the Roman Empire or the fall of Rome, was the loss of central political control in the Western Roman Empire, a process in which the Empire failed to enforce its rule, and its vast territory was divided among several successor polities. The Roman Empire lost the strengths that had allowed it to exercise effective control over its Western provinces; modern historians posit factors including the effectiveness and numbers of the army, the health and numbers of the Roman population, the strength of the economy, the competence of the emperors, the internal struggles for power, the religious changes of the period, and the efficiency of the civil administration. Increasing pressure from invading peoples outside Roman culture also contributed greatly to the collapse. Climatic changes and both endemic and epidemic disease drove many of these immediate factors. The reasons for the collapse are major subjects of the historiography of the ancient world and they inform much modern discourse on state failure.

In 376, a large migration of Goths and other non-Roman people, fleeing from the Huns, entered the Empire. Roman forces were unable to exterminate, expel or subjugate them (as was their normal practice). In 395, after winning two destructive civil wars, Theodosius I died. He left a collapsing field army, and the Empire divided between the warring ministers of his two incapable sons. Goths and other non-Romans became a force that could challenge either part of the Empire. Further barbarian groups crossed the Rhine and other frontiers. The armed forces of the Western Empire became few and ineffective, and despite brief recoveries under able leaders, central rule was never again effectively consolidated.

Theodosius I (Ancient Greek: Θεοδόσιος Theodosios; 11 January 347 – 17 January 395), also known as Theodosius the Great, was Roman emperor from 379 to 395. He won two civil wars and was instrumental in establishing the Nicene Creed as the orthodox doctrine for Nicene Christianity. Theodosius was the last emperor to rule the entire Roman Empire before its administration was permanently split between the Western Roman Empire and the Eastern Roman Empire. He ended the Gothic War (376–382), but did so on terms disadvantageous to the empire, with the Goths remaining and politically autonomous within Roman territory, albeit as nominal allies.

Born in Hispania, Theodosius was the son of a high-ranking general of the same name, Theodosius the Elder, under whose guidance he rose through the ranks of the Roman army. Theodosius held independent command in Moesia in 374, where he had some success against the invading Sarmatians. Not long afterwards, he was forced into retirement, and his father was executed under obscure circumstances. Theodosius soon regained his position following a series of intrigues and executions at Emperor Gratian's court. In 379, after the eastern Roman emperor Valens was killed at the Battle of Adrianople against the Goths, Gratian appointed Theodosius as a successor with orders to take charge of the military emergency. The new emperor's resources and depleted armies were not sufficient to drive the invaders out; in 382 the Goths were allowed to settle south of the Danube as autonomous allies of the empire. In 386, Theodosius signed a treaty with the Sasanian Empire which partitioned the long-disputed Kingdom of Armenia and secured a durable peace between the two powers.

Valens (/ˈveɪlənz/; Ancient Greek: Ουάλης, romanized: Ouálēs; 328 – 9 August 378) was Roman emperor from 364 to 378. Following a largely unremarkable military career, he was named co-emperor by his elder brother Valentinian I, who gave him the eastern half of the Roman Empire to rule. In 378, Valens was defeated and killed at the Battle of Adrianople against the invading Goths, which astonished contemporaries and marked the beginning of barbarian encroachment into Roman territory.

As emperor, Valens continually faced threats both internal and external. He defeated, after some dithering, the usurper Procopius in 366, and campaigned against the Goths across the Danube in 367 and 369. In the following years, Valens focused on the eastern frontier, where he faced the perennial threat of Persia, particularly in Armenia, as well as additional conflicts with the Saracens and Isaurians. Domestically, he inaugurated the Aqueduct of Valens in Constantinople, which was longer than all the aqueducts of Rome. In 376–77, the Gothic War broke out, following a mismanaged attempt to settle the Goths in the Balkans. Valens returned from the east to fight the Goths in person, but lack of coordination with his nephew, the western emperor Gratian (Valentinian I's son), as well as poor battle tactics, led to Valens and much of the eastern Roman army dying in a battle near Adrianople in 378.

The Battle of Adrianople also known as Battle of Hadrianopolis was fought between the Eastern Roman army led by the Roman emperor Valens and Gothic rebels (largely Thervings as well as Greutungs, non-Gothic Alans, and various local rebels) led by Fritigern. The battle took place on 9 August 378 in the vicinity of Adrianople, in the Roman province of Thracia (modern Edirne in European Turkey). It ended with an overwhelming victory for the Goths and the death of Emperor Valens.

As part of the Gothic War of 376–382, the battle is often considered the start of the events which led to the fall of the Western Roman Empire in the 5th century.