The biggest damage to the Roman Empire was done by the Visigoths, who, instead of becoming part of the Roman Army, were never properly assimilated and then began to operate against it. With the attention of Stilicho, left by Theodosius in charge of the Army, occupied by the Visigoths, the Western frontiers were stripped of troops. Stilicho won some battles against the Goths but was never able to repell them completely. In the year 408 Stilicho the magister militum was executed for treason, i.e. for unpermitted colaboration with the Goths.
In the year 407, the Alans, Vandals, and Suevi crossed a frozen Rhine to engage in an uncontested raid through Gaul and Spain. Settling in Spain in 409, these tribes were never troubled by the Romans. Instead, the Visigoths, who soon became semi-independent allies of the Western Emperors despite sacking Rome in 410 and settled in Aquitaine. In fact in 416, the Visigoths broke up the kingdoms of the Alans and the Siling Vandals.
The Suevi became an established Kingdom in Spain, but when the Visigoths expanded from Aquitaine into Spain, the Suevi had to move to the northwest. Their Kingdom survived until the Visigoths completed their conquest of Iberia in 585.
Meanwhile, in 428, the Asding Vandals crossed over into Africa. By 442 they had established themselves, ending the ancient source of grain for Roman Italy. The Visigoths then expanded into the rest of Gaul and Spain (469-478), but they were pushed entirely into Spain by the Franks in 507, absorbing the last Suevi (584), and converting them to Christianity as they converted themselves to Arianism due to Wulfila’s bible translation in the 4th Century. Their Kingdom endured until the armies of Islam arrived in 711.
The origin and history of the Goths is a matter of great interest, dispute, and speculation. But there is no real historical evidence linking the Goths to it, apart from much later, and legendary, accounts, like the history of the Goths completed in 551 by Jordanes, a Goth himself.
What is better known is that in the first centuries A.D. German tribes expanded from the Baltic and North Sea coasts of Germany south and east along the frontier of the Roman Empire. There are strong indication that they were originally coming from Gotland as their language has elements from Swedish and Norse language.
In so doing they interacted with Roman culture, even developing their own writing system, the Runes. By the third century, the Goths were in the forefront of this expansion, passing around the Roman salient of Dacia.
From this position, in 251 the Goths raided into the Balkans, killing the Emperors Decius and Herennius. 267 the Goths sailed down into Roman territory, as the Viking raids of later centuries, sacking Athens. In 269 they were finally defeated at the battle of Naissus by Claudius II, henceforth known as “Gothicus.” Nevertheless, Aurelian then withdrew legions and settlers from Dacia in 271.
By this time some of the Gothic tribes were moving on, and soon two Gothic communities could be distinguished, the Visigoths, in the west, and the Ostrogoths, in the east. These moved onto the steppe of the Ukraine, turned at the Don north, until they reached Baltic Sea.
The Gothic empire of King Ermanaric collapsed abruptly when the Huns arrived in about 370. This pushed the Ostrogoths back into the Visigoths and, in turn, the Visigoths into Roman territory, which began all the troubles for Rome. Soon, the Ostrogoths passed across the Danube also.
The only remaining community the Ostrogoths left behind were located at the Crimea. This survived and was still speaking Gothic as late as the 16th century.