General Theodosius the Younger, who has remained in retirement in Spain following the execution of his father, had married a fellow Spaniard, Aelia Flacilla, at the end of 376.

Theodosius' first son, the future emperor Arcadius, had been born in 377, and his daughter Pulcheria in January 378.

Immediately after the catastrophic defeat and death of Valens at Adrianople, Gratian unexpectedly summons to his court Theodosius to control the incursions.

When Theodosius once again proves his military ability by a victory over the Sarmatians, Gratian, before the army, proclaims him co-emperor on January 19, 379.

The extremely pious Gratian, having come under the influence of Bishop Damasus or Rome and of Bishop Ambrose of Milan, has chosen Theodosius not only for his military ability but also for his orthodoxy.

His dominion is to be the eastern part of the empire, including the provinces of Dacia (present-day Romania) and Macedonia, taken from Gratian's stepbrother, the augustus Valentinian II, which have been especially infiltrated by barbarians in the preceding few years.