Roman

During the fourth century, the Roman emperors commonly employed foederati: irregular troops under Roman command, but organized by tribal structures. To spare the provincial populations from excessive taxation and to save money, emperors began to employ units recruited from Germanic tribes. The largest of these contingents was that of the Goths, who in 382 (376 in some sources), had been allowed to settle within the imperial boundaries, keeping a large degree of autonomy.

In 394 Alaric served as a leader of foederati under Theodosius I in the campaign which crushed the usurper Eugenius. As the Battle of the Frigidus, which terminated this campaign, was fought at the passes of the Julian Alps, Alaric probably learned the weakness of Italy's natural defences on its northeastern frontier at the head of the Adriatic Sea.

Theodosius died in 395, leaving the Empire to be divided between his two sons Arcadius and Honorius, the former taking the eastern and the latter the western portion of the Empire. Arcadius showed little interest in ruling, leaving most of the actual power to his Praetorian Prefect Rufinus. Honorius was still a minor; as his guardian, Theodosius had appointed the magister militum Stilicho. Stilicho also claimed to be the guardian of Arcadius, causing much rivalry between the western and eastern courts.

According to Edward Gibbon in The History of the Decline and Fall of the Roman Empire, during the shifting of offices that took place at the beginning of the new reigns, Alaric apparently hoped he would be promoted from a mere commander to the rank of general in one of the regular armies. He was denied the promotion, however. Among the Visigoths settled in Lower Moesia(now part of Bulgaria and Romania), the situation was ripe for rebellion. They had suffered disproportionately great losses at Frigidus. According to rumour, exposing the Visigoths in battle was a convenient way of weakening the Gothic tribes. This, combined with their post-battle rewards, prompted them to raise Alaric "on a shield" and proclaim him king; according to Jordanes, a 6th-century Roman bureaucrat of Gothic origin who later turned his hand to history, both the new king and his people decided "rather to seek new kingdoms by their own work, than to slumber in peaceful subjection to the rule of others."