The divide of the Roman Empire in a western and an eastern part took place after the death of emperor Theodosius in the year of 395 AD. The encroachment of the Huns to Europe changed the complete political order. Centre of the Hunnic sphere of influence were the Pannonian lowlands at the rivers Danube and Tisza. Only after the costly battle at the Catalaunian Fields (451) and the death of Attila (453) the collapse of the Hunnic kingdom began. Already several decades later (475/480) the Western Roman Empire collapsed. The Eastern Roman (Byzantine) Empire however outlasted additional 1000 years until the middle of the 15th century.
Note: A map of the publication "Die Hunnen" by Timo Stickler (Munich 2007) was used for the delimitation of the Eastern and Western Roman Empire about 450 AD.