Stymied by the Great Walls of the Qin and Tang Dynasties in the early first millennium CE, the Huns moved west. They drove entire nations before them, creating a ripple effect that exacerbated the rot inside the Roman Empire and brought about its fall. When the Huns burst into Europe 400 years later in the mid fifth century, Rome had been sacked three times in fifty years, and what remained of the Western Roman Empire was a Romano-Celtic-Germanic conglomeration at the ripples’ end in Gaul (modern France).
In 450 CE, Honoria, the sister of Roman emperor Valentinian III, was unhappy with her betrothal so she sent a message to the Huns’ leader, Attila, for assistance. Attila took the message as a marriage proposal with the Western Roman Empire as a dowry. Valentinian obviously disagreed. Atilla, known as the Scourge of God because Christians believed that he was sent to punish the corrupt Romans, invaded.
In 451, Attila and his army crossed the Rhine and sacked most of Gaul before confronting a combined Romano-Germanic army under Attila’s friend, Flavius Aetius, and the Visigothic King Theodric I, son of the infamous Alaric, who sacked Rome in 410.
The Allied army was typical of the “barbarization” of the Roman military in the final days of the Roman Empire. Ironically, so was Attila’s. Gone were the days of Roman legionary heavy infantry and Hunnic horse archers, though they existed in small numbers in both armies. Each army was mostly Germanic foederati, light spearmen and horsemen, though the Franks allied with Flavius were already known for the quality of their heavy(er) cavalry. On 20 June 451, outside of Chalons on the Catalaunia Fields, the two nearly indistinguishable armies met. In a chaotic battle in both which commanders lost control, they fought each other to a standstill. Theodric was killed, but Attila felt that no chick was worth this and retreated.
The Battle of Chalons was the last gasp of the Western Roman Empire. Attila’s campaign broke the Roman army, destroyed Gaul, shattered the Visigothic Kingdom, and neutered the Hunnic army. None recovered. Into the vacuum stepped the Slavs and various Germanic nations, especially the Franks. In gratitude for their service (and because they were going to take it anyway) Valentinian gave the semi mythical leader of the Franks, Merovech, land around the town of Aachen as his own. Within 50 years the Merovingian Franks would be the masters of west central Europe.