Adrianople, Battle of, August 9, 378 A.D., a crushing defeat of the Roman imperial army under Emperor Valens by hordes of Visigoths led by Fritigern, aided by Ostrogoths and members of other barbarian tribes. Valens and two-thirds of his army, about 40,000 men, were killed when trapped in a double envelopment. The Battle of Adrianople was the first barbarian victory over the Romans within the empire. It was a turning point in the history of warfare, establishing a dominance of cavalry that was to last 1,000 years.