The East was ruled by his uncle Valens, who was later succeeded by Theodosius I. Gratian subsequently led a campaign across the Rhine, attacked the Lentienses, and forced the tribe to surrender.

That same year, the eastern emperor Valens was killed fighting the Goths at the Battle of Adrianople, which led to Gratian elevating Theodosius to replace him in 379.

[13] When a party of Alamanni visited Valentinian's headquarters to receive the customary gifts towards the end of 364, Ursatius, the magister officiorum made them an offering they considered inferior to that of his predecessor.

[14] Although at first unsuccessful, eventually Jovinus, the magister equitum in Gaul inflicted heavy losses on the enemy at Scarpona (Dieulouard) and at Catalauni (Châlons-sur-Marne), forcing them to retire.

[14] An opportunity to further weaken the Alamanni occurred in the summer of 368, when king Vithicabius was murdered in a coup, and Valentinian and his son Gratian crossed the river Moenus (the Main) laying waste to Alamannic territories.

[15] Valentinian fortified the frontier from Raetia in the east to the Belgic channel, but the construction was attacked by Alamanni at Mount Pirus (the Spitzberg, Rottenburg am Neckar).

In 369 (or 370) Valentinian then sought to enlist the help of the Burgundians, who were involved in a dispute with the Alamanni, but a communication failure led to them returning to their lands without joining forces with the Romans.

Valentinian's decision to establish garrisons across the Danube had angered them, and the situation escalated after the Quadi king, Gabinus, was killed during negotiations with the Romans in 374.

[23] Despite Valentinian being given nominal authority over the praetorian prefectures of Italy, Illyricum, and Africa, Gratian ruled the western Roman empire himself.

Late in July, Valens was informed that the Goths were advancing on Adrianople (Edirne) and Nice, and started to move his forces into the area.

[26] Weeks later, Gratian had arrived in Castra Martis with a few thousand men, by which time Valens was at Adrianople (Latin: Hadrianopolis; Turkish: Edirne).

[27] Encouraged by his advisors to claim victory without sharing the glory with Gratian, as well as being misinformed about the number of enemy troops,[28] Valens attacked the Gothic army and as a result thousands[b] of Romans died in the Battle of Adrianople along with Sebastianus and the emperor himself.

[26][29][13] In the immediate aftermath of Adrianople, Gratian issued an edict of tolerance at Sirmium, restoring bishops exiled by Valens and ensuring religious freedoms to all religions.

[38] Zosimus' report that Gratian refused the robe of office of the pontifex maximus has been doubted by modern scholars, because there is no other mention of such a garment associated with the priesthood.

[43] With the collapse of the Danube frontier[c] under the incursions of the Huns and Goths, Gratian moved his seat from Augusta Treverorum (Trier) to Mediolanum (Milan) in 381.

[44] He became increasingly aligned with the city's bishop, Ambrose, and the Roman Senate, shifting the balance of power within the factions of the western empire.

[57][53] Gratian alienated the army by his favouritism towards his Alan deserters, whom he made his bodyguards and to whom he gave military commands and allowing them to perform human sacrifices to Ares.

[57][61][f] Shortly after, the Roman general Magnus Maximus had raised the standard of revolt in Britain and invaded Gaul with a large army.