By the end of the 1st century AD, Roman governance was established over the entire Balkan Peninsula and Christianity began spreading in the region around the 4th century. The Gothic Bible—the first Germanic language book—was created by Gothic bishop Ulfilas in what is today northern Bulgaria around 381. The region came under Byzantine control after the fall of Rome in 476. The Byzantines were engaged in prolonged warfare against Persia and could not defend their Balkan territories from barbarian incursions. This enabled the Slavs to enter the Balkan Peninsula as marauders, primarily through an area between the Danube River and the Balkan Mountains known as Moesia. Gradually, the interior of the peninsula became a country of the South Slavs, who lived under a democracy. The Slavs assimilated the partially Hellenized, Romanized, and Gothicized Thracians in the rural areas.