military · 378 · Constantinople
Valens ruled the east from 364 and used it: he exiled Nicene bishops, harried the Egyptian monks, and pressed the homoian settlement his brother had inherited from Constantius, backing down chiefly where he met someone like Basil of Caesarea whom he could not move. In August 378 he met the Goths outside Adrianople without waiting for the western army and lost two thirds of the eastern field forces and his own life; the body was never found. Ammianus, who was a soldier, called it the worst defeat since Cannae. The consequence for this dataset is not military. Theodosius was made emperor of the east within five months, he was a Nicene from Spain, and by 381 the council at Constantinople had settled the creed the other way for good. Two generations of imperial pressure for a middle formula ended on a single afternoon, and the settlement that followed has held in both halves of the church ever since.
What does an emperor's death do to a doctrine?
Here, it settled it. Valens was the last Arian emperor of the east, and he died in an afternoon.