It is ironic that the greatest threats to the Roman Empire began not with the West Germanic tribes of the Roman frontier but with the East Germanic tribes, particularly the Goths. The Roman Empire in the early 3rd century had a series of weak emperors and a strong challenge from the Parthian Empire of Persia. The Empire's resources were weakened and the Goths challenged the Romans for control of the area at the mouth of the Danube into the Black Sea. The Goths controlled the area north of the Black Sea while the Romans had conquered territory north of the Danube, which they called Dacia. This is the region of present-day Romania. The attacks of the Goths began in 247 AD and in 251 AD they lured the Roman army under the command of Emperor Decius into a marshy region and defeated it. Emperor Decius was killed in the battle. Encouraged by this victory, the Goths built boats and ships and raided cities on the Black Sea and eastern Mediterranean. The Romans under Claudius I finally managed to finally defeat the Goths in 269 AD and brought peace to the region. Emperor Claudius was thereafter known as Claudius Gothicus. But the government of Claudius Gothicus did not last long and was succeeded by the emperor Aurelian. Emperor Aurelian recognized the reality of the military situation in Dacia and in 270 AD withdrew the Roman troops from Dacia, leaving it to the Goths. The Danube once again became the northern frontier of the Roman Empire in Eastern Europe. In 324 AD Emperor Constantine concluded a treaty with the Visigoths that made them confederates of the Empire, which meant that in exchange for an annual subsidy the Visigoths agreed to help defend the Empire. Nominally Dacia was again counted as part of the Empire but controlled and defended by the Visigoths, confederates of the Roman Empire. The Ostrogoths were located east of the Visigoths in the region beyond the Dniester River. There were significant differences between the Ostrogoths and the Visigoths, generally the Ostrogoths represented a more archaic form of Gothic society. Barbarians' challenges to the Empire imposed a heavy burden on its finances and army. The burden of raising troops and funds to defend the Empire threatened to cause its collapse.