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Before Attila

The history of the Huns in the fourth century is not very clear, and the Huns left no sources themselves. The Romans became aware of the Huns when the latter's invasion of the Pontic steppes forced thousands of Goths to move to the Lower Danube to seek refuge in the Roman Empire in 376. The Huns conquered the Alans, most of the Greuthungi or Eastern Goths, and then most of the Thervingi or Western Goths, with many fleeing into the Roman Empire. In 395 the Huns began their first large-scale attack on the Eastern Roman Empire. Huns attacked in Thrace, overran Armenia, and pillaged Cappadocia. They entered parts of Syria, threatened Antioch, and passed through the province of Euphratesia. At the same time, the Huns invaded the Sasanian Empire. This invasion was initially successful, coming close to the capital of the empire at Ctesiphon; however, they were defeated badly during the Persian counterattack.

During their brief diversion from the Eastern Roman Empire, the Huns may have threatened tribes further west. Uldin, the first Hun identified by name in contemporary sources, headed a group of Huns and Alans fighting against Radagaisus in defense of Italy. Uldin was also known for defeating Gothic rebels who troubled the East Romans around the Danube and for beheading the Goth Gainas around 400–401. The East Romans began to feel the pressure from Uldin's Huns again in 408. Uldin crossed the Danube and pillaged Thrace. The East Romans tried to buy off Uldin, but his sum was too high so they instead bought off Uldin's subordinates. This resulted in many desertions from Uldin's group of Huns. Uldin himself escaped back across the Danube, after which he is not mentioned again in history.

Hunnish mercenaries are mentioned on several occasions being employed by the East and West Romans, as well as the Goths, during the late 4th and 5th century. In 433 some parts of Pannonia were ceded to them by Flavius Aetius, the magister militum of the Western Roman Empire.