Valentinian was born in AD 321 in Pannonia. His father was a certain Gratianus who came from Cibalae in Pannonia. The young Valentinian received a good education, even receiving training in painting and sculpture, arts for which he was to display much talent.
After a military career he came to serve as a military tribune under Constantius II in Mesopotamia (ca. AD 360) and later commanded a troop of spearmen under Julian.
At the death of Jovian, the commanders of the army withdrawing from its Mesopotamian campaign, having reached Nicaea with the main force, held a council with some civic leaders to choose a new emperor. Together they decided on Valentinian, who was still at Ancyra (Ankara) at the time (AD 364).
Had there been previous divisions of the empire into eastern and western parts, then it had always been eventually unified again. This division though between Valentinian and Valens proved to be final. For a short time the empires should run in harmony. And indeed under Theododius they would even be briefly reunited again. Though it was this division which is seen as the defining moment when east and west established themselves as separate realms.
Valentinian chose the western part for entirely unselfish reasons, not seeking the richer and larger part. In fact the east possessed the greater resources. But the threat to the west was by far the greater one and hence Valentinian chose to give it his personal attention.
Valentinian quickly saw the need to strengthen the military in order to deal with the enormous threats arisen in the north. For this he tried to raise the status of the soldiers. Money was hard to come by, so he instead distributed seeds and stock among them allowing them to act as farmers in their spare time to supplement their income.
No sooner had he taken control of the west and set up his headquarters at Mediolanum (Milan), the empire saw a series of barbarian invasions. First to come were the Alemanni, crossing the Rhine and capturing the strategic bastion of Moguntiacum (Mainz). Though they were eventually defeated in three successive battles by Valentinian's 'Master of Horse', Flavius Jovinus.
In the same year, AD 367, Valentinian appointed his elder son Gratian as co-Augustus in the west. It was to prove a highly successful attempt of launching a new dynasty, for the 'House of Valentinian' should be a long lasting one. Even the brief emperor Olybrius in AD 472 should still be a distant descendant of Valentinian and his second wife Justina.
To pay for the great military efforts required during his time, Valentinian saw himself forced to introduce the highest, most oppressive Roman taxes so far. Though he did so reluctantly and with sincere concern for the poor.
In religious matters Valentinian was a Christian, but a leader of far less religious fervour than any of his immediate predecessors. His reign hence was based on a policy of broad religious tolerance.
Valentinian stayed in Germany for no less than seven years, creating elaborate defensive systems along the Rhine and forging allegiances with various barbarian tribes. For example allying himself with the Burgundians who were the traditional enemies of the Alemanni.
The next invasion followed in AD 374 as in the east of Raetia Germans and Sarmatians together crossed the Danube.
In AD 375 Valentinian responded by moving his headquarters to Sirmium. He successfully drove the barbarians back across the river, reinforced the defences, and crossed to the other side of the Danube, devastating the adjourning German territories.