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.

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.

A capable administrator[12] who significantly relieved the burden of taxation on the population,[13] Valens is also described as indecisive, impressionable, a mediocre general and overall "utterly undistinguished".

According to the 5th-century Greek historian Socrates Scholasticus, Valens refused pressure to offer pagan sacrifices during the reign of the polytheist emperor Julian.

[22] The Latin historian Ammianus Marcellinus relates that Valentinian was summoned to Nicaea by a council of military and civil officials, who acclaimed him augustus on 25 February 364.

[25] It was the general opinion that Valentinian needed help to handle the administration, civil and military, of the large and unwieldy empire, and, on 28 March, at the express demand of the soldiers for a second augustus, he selected Valens as co-emperor at the Hebdomon, before the Constantinian Walls.

[34] Recent tax increases,[35] and Valens's dismissal of Julian's popular minister Salutius, contributed to a general disaffection and to the acceptability of a revolution.

[41] Procopius met the danger from the new emperors with his own bid for power, emphasizing his connection to the revered Constantinian Dynasty: during his public appearances he was always accompanied by Constantia, the posthumous daughter of Constantius II, and her mother Faustina, the dowager empress.

Valens recovered his nerve and sent an army to Constantinople; according to Ammianus Marcellinus, the soldiers defected to Procopius, whose use of his Constantinian hostages had met with some success.

[45] According to Ammianus Marcellinus and the later Greek historians Socrates Scholasticus and Sozomen, the forces of Valens eventually prevailed after eight months, defeating Procopius in battles at Thyatira and Nacoleia.

[18] During Procopius's insurrection, the Gothic king Ermanaric, who ruled a powerful kingdom north of the Danube from the Euxine to the Baltic Sea,[48] had engaged to supply him with troops for the struggle against Valens.

The Gothic army, reportedly numbering 30,000 men, arrived too late to help Procopius, but nevertheless invaded Thrace and began plundering the farms and vineyards of the province.

Jovian had surrendered Rome's much disputed claim to control over Armenia in 363, and Shapur II was eager to make good on this new opportunity.

Meanwhile, troubles broke out with the boy-king Pap, who purportedly had the Armenian patriarch Nerses assassinated and demanded control of a number of Roman cities, including Edessa.

Pressed by his generals and fearing that Pap would defect to the Persians, Valens made an unsuccessful attempt to capture the prince and later had him executed inside Armenia.

Valens became the senior augustus on 17 November 375, after his older brother Valentinian died suddenly at Brigetio (Szőny) while on campaign against the Quadi in Pannonia.

After failing to hold the Dniester or the Prut rivers against the Huns, the Goths retreated southward in a massive emigration, seeking new settlements and shelter south of the Danube, i.e. Roman lands, which they may have thought could be held against the enemy.

In 376, the Visigoths under their leader Fritigern advanced to the far shores of the lower Danube and sent an ambassador to Valens who had set up his capital in Antioch, and requested asylum.

The small number of imperial troops present prevented the Romans from stopping a Danube crossing by a group of Ostrogoths and yet later on by Huns and Alans.

[64] In early 377, the Goths revolted after a commotion with the people of Marcianopolis, and defeated the corrupt Roman governor Lupicinus near the city at the Battle of Marcianople.

In a sanguinary battle at Ad Salices, the Goths were momentarily checked,[66] and Saturninus, now Valens's lieutenant in the province, undertook a strategy of hemming them in between the lower Danube and the Euxine, hoping to starve them into surrender.

The Romans then fell back, incapable of containing the irruption, though with an elite force of his best soldiers the general Sebastian was able to fall upon and destroy several of the smaller predatory bands.

The imperial councillors, comes Richomeres, and the generals Frigeridus and Victor cautioned Valens to wait for the arrival of the western army, a course Gratian also recommended in his letters.

The populace of Constantinople was impatient at the delay and its opinion of Valens became hostile: he was criticized for failing to control the Goths after inviting them into his territory, and compared unfavourably with Gratian as a military commander.

[69] According to the Latin historians Ammianus Marcellinus and Paulus Orosius, on 9 August 378, Valens and most of his army were killed fighting the Goths near Hadrianopolis in Thrace (Adrianople, Edirne).

[70] Valens opened the campaign with arrangements aimed at building his troop strength and gaining a toehold in Thrace, then moved out to Adrianople, from whence he marched against the confederated barbarian army.

[46] "Valens was utterly undistinguished, still only a protector, and possessed no military ability: he betrayed his consciousness of inferiority by his nervous suspicion of plots and savage punishment of alleged traitors," writes A. H. M. Jones, a modern historian.

This work, produced by Valens's secretary Eutropius, and known by the name Breviarium ab Urbe condita, tells the story of Rome from its founding.

[80] Toward the end of his Res Gestae (XXXI.14.7), Ammianus says that Valens was physically compact, dark-complected, and of average height, "knock-kneed, and somewhat pot-bellied", and had a "dimmed" pupil in one eye (the translator John C. Rolfe suggests that this is a description of a cataract).