The role of choosing a new Augustus fell again to army officers. On 28 February 364, Pannonian officer Valentinian I was elected Augustus in Nicaea, Bithynia. However, the army had been left leaderless twice in less than a year, and the officers demanded Valentinian choose a co-ruler. On 28 March Valentinian chose his own younger brother Valens and the two new Augusti parted the Empire in the pattern established by Diocletian: Valentinian would administer the Western Roman Empire, while Valens took control over the Eastern Roman Empire.

Valens's election would soon be disputed. Procopius, a Cilician maternal cousin of Julian, had been considered a likely heir to his cousin but was never designated as such. He had been in hiding since the election of Jovian. In 365, while Valentinian was at Paris and then at Rheims to direct the operations of his generals against the Alamanni, Procopius managed to bribe two legions assigned to Constantinople and take control of the Eastern Roman capital. He was proclaimed Augustus on 28 September and soon extended his control to both Thrace and Bithynia. War between the two rival Eastern Roman Emperors continued until Procopius was defeated. Valens had him executed on 27 May 366.

On 4 August 367, a third Augustus was proclaimed by the other two. His father Valentinian and uncle Valens chose the eight-year-old Gratian as a nominal co-ruler, obviously as a means to secure succession.

In April 375 Valentinian I led his army in a campaign against the Quadi, a Germanic tribe which had invaded his native province of Pannonia. During an audience with an embassy from the Quadi at Brigetio on the Danube, a town now part of modern-day Komárno, Slovak republic, Valentinian suffered a burst blood vessel in the skull while angrily yelling at the people gathered. This injury resulted in his death on 17 November 375.

Succession did not go as planned. Gratian was then a 16-year-old and arguably ready to act as Emperor, but the troops in Pannonia proclaimed his infant half-brother emperor under the title Valentinian II.

Gratian acquiesced in their choice and administered the Gallic part of the Western Roman Empire. Italy, Illyria and Africa were officially administrated by his brother and his stepmother Justina. However the division was merely nominal as the actual authority still rested with Gratian.
Battle of Adrianople

Meanwhile, the Eastern Roman Empire faced its own problems with Germanic tribes. The Thervingi, an East Germanic tribe, fled their former lands following an invasion by the Huns. Their leaders Alavivus and Fritigern led them to seek refuge in the Eastern Roman Empire. Valens indeed let them settle as foederati on the southern bank of the Danube in 376. However, the newcomers faced problems from allegedly corrupted provincial commanders and a series of hardships. Their dissatisfaction led them to revolt against their Roman hosts.

For the following two years conflicts continued. Valens personally led a campaign against them in 378. Gratian provided his uncle with reinforcements from the Western Roman army. However this campaign proved disastrous for the Romans. The two armies approached each other near Adrianople. Valens was apparently overconfident of the numerical superiority of his own forces over the Goths. Some of his officers advised caution and to await the arrival of Gratian, others urged an immediate attack and eventually prevailed over Valens, who, eager to have all of the glory for himself, rushed into battle. On 9 August 378, the Battle of Adrianople resulted in the crushing defeat of the Romans and the death of Valens. Contemporary historian Ammianus Marcellinus estimated that two thirds of the Roman army were lost in the battle. The last t
hird managed to retreat.

The battle had far-reaching consequences. Veteran soldiers and valuable administrators were among the heavy casualties. There were few available replacements at the time, leaving the Empire with the problems of finding suitable leadership. The Roman army would also start facing recruiting problems. In the following century much of the Roman army would consist of Germanic mercenaries.

For the moment however there was another concern. The death of Valens left Gratian and Valentinian II as the sole two Augusti. Gratian was now effectively responsible for the whole of the Empire. He sought however a replacement Augustus for the Eastern Roman Empire. His choice was Theodosius I, son of formerly distinguished general Count Theodosius. The elder Theodosius had been executed in early 375 for unclear reasons. The younger one was named Augustus of the Eastern Roman Empire on 19 January 379. His appointment would prove a deciding moment in the division of the Empire.