When Valentinian died, Gratian ruled in the west and his uncle Valens ruled in the east. Gratian had been fighting the Alamanni while Valens was dealing with the Goths at Adrianople. Gratian was scheduled to come to Valens' assistance, but arrived too late to prevent the disaster, whether because he was actually too late or Valens had jumped the gun in order to take the credit for what he wrongly assumed would be victory.
The disaster at Adrianople is one of the major turning points in the fortunes of Imperial Rome and a possible date for Rome's fall.
Read more about Gratian.