Valens next waged war on the Visigoths (Visigoth), who had aided Procopius and were threatening to invade Thrace. In May 367 the Emperor crossed the Danube and devastated the Visigothic territories (in modern Romania). Two years later he invaded the area again and decisively defeated the tribe. After suppressing the conspiracy of Theodorus at Antioch in the winter of 371–372, Valens became involved in war with the Persians. He achieved a victory in Mesopotamia but in 376 was obliged to make peace on unfavourable terms. In that year the Visigoths, defeated and pursued by the Huns, were allowed by Valens' generals to settle in Roman territory south of the Danube. Soon the tribe rebelled against the Romans and engaged the Emperor in the great Battle of Adrianople (Adrianople, Battle of) (modern Edirne, Tur.) on Aug. 9, 378. The poor tactics employed by Valens led to the total defeat of his army, and the Emperor himself was numbered among the fallen.

Valens was an Arian Christian who persecuted Catholics while interfering little with the pagans. Bishops who had been restored by the emperor Julian were banished, although near the end of his reign Valens relented somewhat and allowed these exiles to return.