In one of the most decisive battles in history, a large Roman army under Valens, the Roman emperor of the East, is defeated by the Visigoths at the Battle of Adrianople in present-day Turkey. Two-thirds of the Roman army, including Emperor Valens himself, were overrun and slaughtered by the mounted barbarians.
Crowned in 264, Emperor Valens initiated warfare against the semi-civilized Visigoths in 364 and by 369 had defeated them. Visigoths under Fritigern were given permission to settle south of the Danube in the Roman Empire but, subjected to oppressive measures by Roman officials, they soon rose in revolt. In 378, Valens marched a Roman army against Fritigern, and 10 miles from Adrianople the Romans came upon the massed barbarians. As the Visigoth cavalry was off on a forging mission, Valens ordered a hasty attack on August 9. The Romans initially drove the barbarians back, but then the Visigoth cavalry suddenly returned, routing the Romans and forcing them into retreat. The horsemen then rode down and slaughtered the fleeing Roman infantry. Some 20,000 of 30,000 men were killed, including Emperor Valens.
The decisive Visigoth victory at the Battle of Adrianople left the Eastern Roman Empire nearly defenseless and established the supremacy of cavalry over infantry that would last for the next millennium. Emperor Valens was succeeded by Theodosius the Great, who struggled to repel the hordes of Visigoth barbarians plundering the Balkan Peninsula.