- 378 A.D.
Valens was the younger brother of Valentinian who made him co-emperor soon after his own accession. He then sent Valens off to take care of the eastern portion of the empire. Making Constantinople his capital, he dealt with the Persian factor as best he could which wasn't all that great and saw his borders shrink as a consequence. His ineptitude showed most compellingly at the battle of Adrianople in 378. Resettled barbarians had been allowed within the nominal borders of the empire but were allowed to keep their arms. As the barbarians became squeezed from the double whammy of external tribes pushing against their lands and the systematic and extra-official hard-line policies of lower Roman government officials they became aggressive once more and aimed to push southward.
Near the city of Adrianople they gathered with the intention of making war and Valens prepared to meet them in battle. He sent for reinforcements from Gratian but before these could arrive he figured in a bout of short-sighted arrogance that he could go at it alone and claim the glory all to himself afterward. On an exceedingly poorly executed attack plan the Romans attacked the barbarians. In what apparently was meant to be a surprise attack, he rushed the barbarians before getting his soldiers into formation and before they had eaten. It was also a very hot and dusty day making their attack that much more energy-sapping. The barbarians watched as their counterparts fumbled and figured out their strategy. Their general sent his cavalry in an outflanking maneuver and managed to encircle Valens's infantry and subsequently slaughtered most. Valens himself perished in battle and the loss of the army along with the demoralizing effect to the rest of the military was another step in the inexorable fall of the empire almost one hundred years later.