Procopius – Gibbon’s Decline and Fall of the Roman Empire Chapter 25 Part 2
Valentinian and his brother Valens were both emperors but were not really equal partners, Valens was very much carried along in Valentinian’s coat tails. So it was Valentinian that was responsible for dividing the empire in two. The empire had been carved up in various ways for diverse reasons over its history so far. There was no particular reason why anyone, least of all Valentinian himself would have suspected that the division made in 364 would prove to be permanent. But with hindsight it is easy to see that it was in fact inevitable.
