Excerpts from Books and Wikipedia
"But the younger Valens belonged to the Arian branch of Christianity, and he was entirely intolerant of any other form of doctrine. He began a war of extermination against the Nicene Christians in Antioch: exiling their leader, driving out the followers, and drowning some of them in the Orontes. . . . Valens received an official delegation of Goths asking permission to settle in the Roman land on the other side of the Danube. . . . With the dam of the Roman border breached, new waves of fleeing Goths poured across the Danube. . . . Within two years, Valens’s decision led, yet again, to war with the barbarians. . . . On August 9, 378, Valens plunged into the battle among his men and was killed. Two-thirds of his army fell with him;" [Bauer: Medieval World, p. 42-9]
"He was given the eastern half of the empire by his brother Valentinian I after the latter's accession to the throne. Valens was defeated and killed in the Battle of Adrianople, which marked the beginning of the collapse of the Western Roman Empire." [Wikipedia]