King of the Visigoths (369-380)

Fritigern (died 380), King of the Visigoths (369-380), was the first Germanic king to win a lasting victory over a Roman military force in more than 350 years. 

A rival of Athanaric, another prominent Visigothic leader, Fritigern (Gothic Frithugairns, ironically meaning "desiring peace") was favored by the Roman Emperor Valens primarily because of his adoption of Arianism, a form of Christianity popular at the time and one which Valens sympathized with. In 376, the Visigoths, under heavy pressure from the Huns who had already conquered their kinsmen, the Ostrogoths, asked Valens to allow them to cross the Danube River and settle in Roman territory, which the Huns could not then reach due to their inability to cross the Danube in force. 

Valens agreed to permit Fritigern's followers to enter the empire. In return, they would be subject to military service, but would be treated the same as other Roman subjects. As it turned out, neither happened. During the fall of 376, the Romans helped Fritigern's people cross the Danube and settle in the province of Moesia. However, many followers of Athanaric also slipped across the river, which increased Fritigern's problems in governing his people. 

In 377, a famine hit the areas settled by the Visigoths, and their appeals for help went unanswered. In fact, the Roman governors of the area, Lupicinus and Maximus, treated them badly and forced thousands of them into slavery. With no other choice but to fight or die, Fritigern led his people into battle, and the Visigoths soon held sway over much of the neighboring, richer province of Thrace. The crisis continued into 378, and on August 9 of that year, Fritigern avenged his kinsmen's defeat of 109 years before at the Battle of Naissus by handing Rome its worst military defeat in centuries, at the Battle of Adrianople. 

Fritigern's victory soon led to the Visigoths gaining control of nearly the entire Balkan peninsula. Although his army lacked the siege instruments needed to take the Roman capital of Constantinople, they did raid Greece, leaving only small areas of the country unravaged, including the city of Athens. 

Fritigern continued to battle the Romans with mixed success for two years after his great victory, ultimately winning recognition as king by most of the Visigoths within the empire. When he died, Athanaric became king of the entire Visigothic peoples and finally made peace with the Romans.

Note: Some of you will notice this text is almost exactly the same as that in the Wikipedia. Before you accuse me of stealing from them, please be advised that 1) the Wikipedia is an open-source project; and 2) I wrote the entire original article on that site myself.