Parts of the eastern Balkan Peninsula were in antiquity inhabited by the Thracians who were a group of Indo-European tribes. The whole region as far north as the Danube River was gradually incorporated into the Roman Empire by the 1st century CE. The decline of the Roman Empire after the 3rd century CE and the continuous invasions of Goths and Huns left much of the region devastated, depopulated and in economic decline by the 5th century. The surviving eastern half of the Roman Empire, called by later historians the Byzantine Empire, could not exercise effective control in these territories other than in the coastal areas and certain cities in the interior. Nonetheless, it never relinquished the claim to the whole region up to the Danube. A series of administrative, legislative, military and economic reforms somewhat improved the situation but despite these reforms disorder continued in much of the Balkans. The reign of Emperor Justinian I (r. 527–565) saw temporary recovery of control and reconstruction of a number of fortresses but after his death the empire was unable to face the threat of the Slavs due to the significant reduction of revenue and manpower.