How did the Battle of Adrianople in 378 lead to Theodosius I becoming emperor?

The Battle of Adrianople on August 9, 378, was a catastrophic defeat for the Roman Empire, resulting in the death of the Eastern emperor Valens and the destruction of two-thirds of his army at the hands of the Visigoths.1

This military disaster created an immediate crisis of leadership in the East. Following Valens's death, the Western emperor Gratian was left to manage the empire's defense. Recognizing the need for a capable military leader to stabilize the situation, Gratian summoned Theodosius I to his court.2

Theodosius had previously served as a military commander in Moesia, where he had demonstrated his competence by defeating the Sarmatians. After Theodosius proved his military ability once more by securing another victory against the Sarmatians, Gratian proclaimed him co-emperor on January 19, 379. Theodosius was subsequently granted dominion over the Eastern part of the empire, which had been severely compromised by the recent barbarian incursions.2