The latest of Ian Hughes' Late Roman biographies here tackles the careers of the brother emperors, Valentinian and Valens. Valentian was selected and proclaimed as emperor in AD 364, when the Empire was still reeling from the disastrous defeat and death in battle of Julian the Apostate (363) and the short reign of his murdered successor, Jovian (364). With the Empire weakened and vulnerable to a victorious Persia in the East and opportunistic Germanic tribes along the Rhine and Danube frontiers, not to mention usurpers and rebellions within, it was not an enviable position. Valentian decided the responsibility had to be divided (not for the first or last time) and appointed his brother as his co-emperor to rule the eastern half of the Empire. 
Valentinian went on to stabilize the Western Empire, quelling revolt in North Africa, defeating the 'Barbarian Conspiracy' that attacked Britain in 367 and conducting successful wars against the Germanic Alemanni, Quadi and...