Many historians view the defeat of Rome in the 4th century war against the Visigoths with the fall of the Western half of the Roman Empire. It certainly didn't help.
Some have called the Roman Emperor Valens, who reigned from A.D. 364-378, The Last True Roman. Perhaps this is a fitting title for the man who failed to save the struggling Western Roman Empire. Those who are familiar with ancient Roman history may recognize the time of Valens’ reign as a time of great war and strife between the Empire and the barbarian tribes of the North, particularly the Visigoths (referred to simply as Goths for short). Now, the Goths had learned their lesson 32 years prior to Valens' arrival in Rome, when they were defeated by Rome the first time, under Emperor Constantine. These Goths, apart from their barbaric and cruel ways, are to be commended for this. They understood that they could not defeat the Roman Empire, so they didn't attack it anymore.
These Goths, under the rule of King Athanaric, seemed to be trying their best to stay out of the way of the Romans at this point, but it appears that Valens had had something to prove, because in 367, fearing that the Goths were preparing their own uprising (though this was never proven to be true), Valens crossed the Denube (the river which formed the northern boundary of the Empire at this point) and attacked them, causing the Goths to flee to the Northeast. Two years later, in 369 Athanaric surrendered on behalf of the Goths, forming a treaty with Valens that for the most part nullified any relations between the two groups (at least for the next few years). Valens, under normal circumstances, probably wouldn’t have agreed to the treaty, and would have stayed and finished killing off the Goths, but in his absence from Rome, the state of affairs there had begun to plummet, necessitating a speedy return.
Fast forward a few years, to A.D. 375, when the Goths were banished from their homeland after an attack from the Huns. About 200,000 of these barbaric people were desperate enough to camp on the north side of the Danube being polite enough not to cross into the Roman Empire without permission
From there the Goths sent a delegation to Rome in order to plead with Valens to let them cross over and make their camps south of the river, where they might be sheltered from any further attacks from the Huns.
Valens, in a fit of shrewdness, agreed wholeheartedly. In fact, he not only agreed to their crossing over, but he offered them good farmland as well. He sent several of his troops north in order to help oversee the settlement.
It was all an excellent show of magnanimity. At least on the surface.
But of course, Valens had more on his mind than simple charity. For his eyes did not see 200,000 helpless settlers camping in his yard. He saw 200,000 potential Roman soldiers, which could swell his badly deflated army (for the Roman Empire was already in its decline even then).
As soon as Valens opened the doors to the Goths, the dam (figuratively speaking) burst. It seemed that every Goth in the world began to make their way into the Empire, and the settlement was a disaster from the very start. First off, Valens had decided not to allow just anyone into his Empire. He attempted to let only those families in which there were men who might potentially be soldiers in, leaving the poor, the sick and the elderly behind to the Huns. It didn't help matters that those soldiers who were supposed to be overseeing the settlement began to mistreat their visitors in some pretty horrible ways which understandably upset the Goths.
With that many people settling in such a small region, famine began to take its toll almost as soon as they had arrived. In order to get food, the Goths had no choice but to begin plundering the Roman lands immediately surrounding them, which didn't earn them much respect from their landlords.
Some might say that this was the beginning of the end for Emperor Valens. He led some troops up North from Constantinople (the capital city in the Eastern Roman Empire) to stop the Goths from the free reign they felt they had over the country. In 378, Valens attacked the Goths in the battle of Adrianople, where he himself, along with many others, died, and the Goths claimed victory. This battle has been viewed as being the final blow for at least the western half of the Roman Empire (the Eastern, or Byzantine, Empire would continue for more than a millennium).
The war between the Romans and the Goths continued for another five years (Valens was promptly replaced by another Emperor, Theodosius I), until finally the Roman armies from both the East and West had pushed the Goths back as far as Thrace (present day Turkey). Finally, on October 3rd, 381, peace was formally made. The Goths had been defeated once again, though they had taken Emperor Valens with him.
The Goths would return thirty years later to give Rome a good sacking in A.D. 410 (though the western Empire was already in shambles at this point), and after this, the Visigoth Kingdom would rise up and control most of the Iberian Peninsula (present day Portugal, Spain and France) until the 7th century.
Surely the war with Rome had earned the Visigoths some confidence and credibility with other armies, which enabled their success in Western Europe, which helped them establish their foothold in the old Roman lands. Still, Visigoth cities and buildings have survived to this day in the lands once controlled by them, so the legacy of the Goths lives on.
References:
Gibbon, Edward. "The Decline and Fall of the Roman Empire." 1789.
The Catholic Enclyclopedia. “Visigoths.” http://www.newadvent.org/cathen/15476b.htm