The Visigoths had previously been allies of the Romans. In fact, it was as allies that they were allowed to settle in the eastern half of the Empire. But this came with a catch. In return for land and provisions, the Visigoths would provide soldiers for the Roman army. What's more, they would have to make their children available as slaves. And in a final humiliation, the Visigoths were required to hand over their weapons, an intolerable demand for such a proud martial race.
It was no surprise, then, when the Visigoths not only refused the Romans' unacceptable terms, but rose up in rebellion against them, along with other barbarian tribes. The Romans tried to nip the rebellion in the bud by assassinating its ring-leaders, but all to no avail. This left the Goths free to rampage through the countryside, pillaging whatever they could get their hands on.
Under the circumstances, it was inevitable that some kind of armed conflict would break out between the Romans and the Goths, and that's precisely what happened. Matters came to a head in 378 at the Battle of Adrianople, at which the Goths inflicted a catastrophic defeat upon Emperor Valens's Roman legions.
In one of the worst calamities of Roman military history, somewhere in the region of 10,000 troops were killed in battle. The defeat underlined the weakness of the Roman military and encouraged other barbarian tribes to use force to get what they wanted. The Battle of Adrianople can be seen as a turning point in the history of Rome and another stage on the road to the eventual destruction of the western half of the Roman Empire.