Hun attack on the Roman Empire
No one knows exactly where the Huns came from. Some scholars believe they originated from the nomad Xiongnu people who entered the historical record in 318 B.C. and terrorized China during the Qin Dynasty and during the later Han Dynasty. The Great Wall of China was reportedly built to help protect against the mighty Xiongnu.
By 370 AD, the Huns had arrived on the Volga, and by 430, they had established a vast, if short-lived, dominion in Europe, conquering the Goths and many other Germanic peoples living outside of Roman borders and causing many others to flee into Roman territory.