structural:scholar_edition, information:extraction
Career

Theodosius is first attested accompanying his father to Britain on his expedition in 368–369 to suppress the " Great Conspiracy ", a concerted Celtic and Germanic invasion of the island provinces. After probably serving in his father's staff on further campaigns, Theodosius received his first independent command by 374 when he was appointed the dux (commanding officer) of the province of Moesia Prima in the Danube. In the autumn of 374, he successfully repulsed an incursion of Sarmatians on his sector of the frontier and forced them into submission. Not long afterwards, however, under mysterious circumstances, Theodosius's father suddenly fell from imperial favor and was executed, and the future emperor felt compelled to retire to his estates in Hispania.

Although these events are poorly documented, historians usually attribute this fall from grace to the machinations of a court faction led by Maximinus, a senior civilian official. According to another theory, the future emperor Theodosius lost his father, his military post, or both, in the purges of high officials that resulted from the accession of the 4-year-old emperor Valentinian II in November 375. Theodosius's period away from service in Hispania, during which he was said to have received threats from those responsible for his father's death, did not last long, however, as Maximinus, the probable culprit, was himself removed from power around April 376 and then executed. The emperor Gratian immediately began replacing Maximinus and his associates with relatives of Theodosius in key government positions, indicating the family's full rehabilitation, and by 377 Theodosius himself had regained his command against the Sarmatians.

Theodosius's renewed term of office seems to have gone uneventfully, until news arrived that the eastern Roman emperor, Valens, had been killed at the Battle of Adrianople in August 378 against invading Goths. The disastrous defeat left much of Rome's military leadership dead, discredited, or barbarian in origin, to the result that Theodosius, notwithstanding his own modest record, became the establishment's choice to replace Valens and assume control of the crisis. With the begrudging consent of the western emperor Gratian, Theodosius was formally invested with the purple by a council of officials at Sirmium on 19 January 379.