At the fall of his father, Theodosius the "dux" of Moesia Prima retired to his estates in the Iberian Peninsula, where he married Aelia Flaccilla in 376. Their first child, Arcadius, was born around 377. Pulcheria, their daughter, was born in 377 or 378. Theodosius had returned to the Danube frontier by 378, when he was appointed "magister equitum". following his successes in the field elevated at Sirmium (Sremska Mitrovica) to the rank of "augustus" by the emperor Gratian () on 19 January 379.