|When barbarians are at the gate but you gotta strut your stuff.|
Last we checked, the various barbarian tribes were kicked out of the empire by Emperor Diocletian. In 376, the Goths—perhaps wanting to revisit such an entente— appeared along the Danube River to petition the emperor of the eastern half of the Roman Empire (Valens) to cross the river and enter the empire; the goths were fleeing the Huns and, in a way, were refugees from the marauding violence of the Huns. Surprisingly, Valens allows the Goths, around hundred-thousand, to enter the empire—with certain conditions; firstly, he will not allow all the Goths to enter simply because there is too many of them; secondly, the emperor requires that all Goths convert to Christianity upon entering (which many do within a few decades), though they end up converting to Aryanism instead of Catholicism, which will cause future trouble; thirdly, the Goths are required to perform military service for the empire.
In theory, this was a good plan. As with all good plans, though, it broke down. Why it broke down can be blamed to the local Roman officials who tried to take advantage of the Goths’ situation by withholding food and selling it for exorbitant prices. Sometimes these officials even demanded that certain family members be incarcerated as slaves before the food shipments were released. The Roman officials threw oil on the fire after a failed attempt to capture the Goth leadership; some of the leaders escaped and the horde turned violent.
In 378, Valens leads the Roman army against the goths but suffers a devastating defeat. How bad? Well, he is killed, so pretty bad. Eventually, though, in 382, a peace treaty is signed between the Goths and the romans; after all, the Goths are in no position to take over the Roman Empire. The treaty essentially restores the status quo of 376 where the Goths live south of the Danube and perform military service on behalf of the Romans.
Roman-Barbarian relations, however, would deteriorate; due to both ambitions on behalf of certain Goth leaders, as well as general anti-Barbarian sentiment, great social discord spread; the Goths would occasionally sack a Roman town if their demands were not met, while the occasional massacre of Goths would take place when local Romans thought that Goths had overstepped their boundaries. One such major massacre—orchestrated by Roman senators— happened in 408 and the retaliatory strike from the Goths would ultimately lead to the sack of Rome in 410.
In 406-7, it was an especially cold winter. Along the Rhine River, where groups like the Vandals were kept outside Roman territory, the river actually froze over, thus allowing the Vandals to take advantage of the depleted guardhouses—the guards were relocated to other places in the Empire in fear of a Goth invasion—to cross over into the Empire. Unfortunately, the Empire did not have enough man-power to deal with this new incursion; so they were forced to make peace with the Goths in 418 in order to use them to turn back the Goths.
This new treaty made for far better terms with the Goths, who now had the ability to settle in the highly valuable locale of southern France, where they were supported by tax revenues, and to elect their own king to act as an intermediary. They still had to perform military service, but they were no longer stuck within the Danube River frontier. But Roman policy, of course, is to play barbarian groups against one another, changing sides as one faction grows weaker and another stronger. This makes sense since in the fifth century, the Roman Empire is rapidly contracting around the core Italian centers in response to continued pressure from the various barbarian groups, abandoning outlying areas: in 409, the Roman Army pulled out of England, in Spain the Roman army leaves in 411, never to return in any substantial force, while in 432 Rome is even forced to withdraw from North Africa. Ultimately, though, this policy of playing barbarian groups against one another is not enough to save Rome from a second sack. In 452, the Huns, under their leader Attila, approach and nearly sack the city, but is saved at the last moment due to his death. A different group, however, the Vandals, do sack Rome after the Huns withdrawal.
Eventually, the Goths overtake all of France and proclaim an independent kingdom free of any Roman political influence. This is the start of the end of the Roman Empire and soon other barbarian tribes are carving out their own spheres of influence, ultimately disposing even emperors themselves. As a political unit, the Roman Empire collapses around 476 A.D.