On this date in 364AD, one Flavius Valentinianus, age 42, was proclaimed emperor on the death of the short-lived emperor Jovian.
Valentinian I, as he became known, had impeccable military chops; his dad was a general and he grew up an army brat. He'd been in since he was eighteen and worked his way up through the officer ranks, serving everywhere from Britannia to Africa to Mesopotamia. A senior commander in the armies of Julian the Apostate, he'd been mentored by the best, and when he took the reins of empire, he immediately set about aggressively beating back the barbarian threats on the Rhine and Danube frontiers.
The problem was that the tradition of using co-rulers to help manage the sprawling empire was well underway, and Valentinian needed one to keep an eye on the east while thumped barbarian hordes in the west. He picked his brother, Valens.
While Valentinianus had been off covering himself in blood and glory as a man among men, commanding the legions of Rome, his brother had been... Well, tending the family estates is honest work, nobody's saying it's not, but it probably didn't adequately prepare young Valens for the challenges he was going to face as Official Eastern Imperial Sidekick.