After the death of Valens, Gratian announced Theodosius as emperor of the east.
Theodosius did his best to unite the Christians of the empire. He tried to do this by turning to law. He announced Nicene Christianity as the one, true faith, and in doing this formed a single, unified, catholic church. He threatened to expel heretics, those who did not hold the Nicene Creed, but never actually carried the threats out.
He also had to fight the Goths while all this was going on, and Gratian had even given him some of his own Goth-infested land to deal with. Theodosius chose a strategy of making his army stronger by recruiting barbarians from some regions to fight against others in other regions.
In 382, after four years of fighting, he made a treaty with the Goths. The treaty allowed them toe exist and rule under their king within the borders, but the king had to listen to the emperor. And when they fought, the Goths would be allies, not regular Roman soldiers.
He also had a large part in the Battle of Frigidus, though he died the year after the battle.