On the 9th August 378AD the Romans suffered a devastating defeat at the hands of their former allies, the Visigoths. The Gothic commander was Fritigern. The battle was fought at Adrianople, in Thrace. The Roman Emperor, Valens, was killed along with his bodyguard.
He had thought to crush the smaller Gothic army while in its encampment. Instead his Cavalry was routed when the Gothic Cavalry, which had been away on a foraging party, fortuitously turned up on the battlefield and drove off both the Roman left and right Cavalry. It then turned on the Roman Infantry’s flanks, until the centre was surrounded. By this point many had already fled the field.
Two thirds of the Roman Eastern field army was killed. The Visigoth army then moved on Constantinople. Here they were denied by its walls. The newly created Eastern Roman Emperor, Theodosius, who had been a General, made peace with the Goths three years later. This time they would remain allied in exchange for lands in Thrace.
Meanwhile, the Western Roman Emperor Gratian was fighting the Vandals in Gaul.