In response to a riot he massacred much of the populace of Thessalonica but was forced to do penance by Ambrose of Milan.
In the last years of his reign he put down the usurper Eugenius defeating him at the River Frigidus while earlier he had defeated the usurper Magnus Maximus.  Although he had gone into retirement following the execution of his father, two years later he was recalled to service by Gratian after the death of Valens at Adrianople in 378.  In response to a riot he massacred much of the populace of Thessalonica but was forced to do penance by Ambrose of Milan. Upon his death the Roman Empire was split into Eastern and Western halves by his sons Arcadius and Honorius.  FTP, name this emperor who made Christianity the official religion of the Roman state.
In the last years of his reign he put down the usurper Eugenius defeating him at the River Frigidus while earlier he had defeated the usurper Magnus Maximus.  
Theodosius the Great or Theodosius I