Of the death of Gratianus and the sovereignty of Maximus
Gratianus in the midst of his successes in war and wise and prudent government ended his life by conspiracy. He left no sons to inherit the empire, and a brother of the same name as their father, Valentinianus, who was quite a youth. So Maximus, in contempt of the youth of Valentinianus, seized the throne of the West.
Source: Ecclesiastical History (New Advent)