During the late 300's, Rome was suffering the beginning of the mass migration of Germanic tribes, most as a result of the Hun's invasions. By this time, Rome had split into two empires: the Eastern Empire at Constantinople, and the Western empire at Rome, Italy. The Emperor of the Eastern empire was Valens, a man with no experience with germanic tribes or their way of life.
Valens allowed the Visigoths to settle in the area by the Black Sea, known as Thrace, along the Danube river. The Visigoths under law were not allowed to own weapons as part of the deal, but in reality no tribe could survive anywhere without a defensive force. Therefore, the Visigoths created a rather large army, part infantry, part calvary. Why the Visigoths chose a large heavy calvary force is unknown. Whatever the reason, it proved to be an idea of vital importance.
Towards the 390's AD, more Goths began migrating towards the East. The Ostrogoths, or East Goths, flooded this area, and later forced their Visigoth cousins to migrate agian West in the 400's. These Ostrogoths were more agressive, and fights often broke out between the Romans and the Goths.
In 378 AD, Emperor Valens decided to crush the rebellious Visigoths. He gathered an army that totaled 60,000. 20,000 were Roman cavalry, and the other 40,000 were the famed Roman infantry. Up to this time the Roman infantry were well known as a powerful foot force, armed with heavy armor. Sadly, the cavalry was a light force, with plenty of armor but of little offensive power. They had hoped to defeat the Goths at Adrianople, where a large army assembled. The Goths only numbered 50,000 light infantry and were thought to be no problem for the legions.
The battle began on August 8th*, 378. At first the battle went according to Roman plans. The gothic infantry was badly outnumbered and out armed, and therefore were deafeated in numerous engagments. Valens had thought victory was at hands, despite the fact that the help he had asked for never came. Valens requested aid from Gratian, the Roman emperor of the West, but it never came. Either it was too slow to travel, or Valens was too impatient to wait, the Western Romans never came. This blunder would soon be apperent to the amry of Constantinople.
Sometime during the day, 50,000 Visigoth horsemen joined the fray. The now outnumbered Roman infantry couldn't hold off the cavalry of the Visigoths, and were forced to retreat. The Romans were slautered in the retreat as they became scattered and leaderless. Emperor Valens himself was killed in the battle. In the end, 40,000 Romans died. The number of dead Visigoths is unknown.
Afterwards, the Visigoths tried to take cities close to them, such as Adrianople. The city never fell, yet the Goths still caused considerable trouble. It took negotiatons by Theodosius I to pacify the Goths, though the peace was not an easy one.
*The date has been changed by a number of sources. Some say Aug. 8th, some say Aug. 9th. The date I provoide may not be accurate, so please don't take it as the truth.