The Huns were a nomadic group of people who are known to have lived in Eastern Europe, the Caucasus, and Central Asia between the 1st century AD and the 7th century. They were first reported living east of the Volga River, in an area that was part of Scythia at the time; the Huns' arrival is associated with the migration westward of a Scythian people, the Alans. They were first mentioned as Hunnoi by Tacitus. In 91 AD, the Huns were said to be living near the Caspian Sea and by about 150 AD had migrated southeast into the Caucasus. By 370 AD, the Huns had established a vast, if short-lived, dominion in Europe. In the 18th century, the French scholar Joseph de Guignes became the first to propose a link between the Huns and the Xiongnu people, who were northern neighbours of China in the 3rd century BC. Since Guignes' time, considerable scholarly effort has been devoted to investigating such a connection. However, there is no scholarly consensus on a direct connection between the dominant element of the Xiongnu and that of the Huns. Priscus, a 5th-century Roman diplomat and Greek historian, mentions that the Huns had a language of their own; little of it has survived and its relationships have been the subject of debate for centuries. Numerous other ethnic groups were included under Attila's rule, including very many speakers of Gothic (East Germanic), which some modern authors describe as a lingua franca. Their main military technique was mounted archery. The Huns may have stimulated the Great Migration, a contributing factor in the collapse of the Western Roman Empire. They formed a unified empire under Attila the Hun, who died in 453 AD; their empire broke up the next year. Their descendants, or successors with similar names, are recorded by neighbouring populations to the south, east and west as having occupied parts of Eastern Europe and Central Asia approximately from the 4th century to the 6th century. Variants of the Hun name are recorded in the Caucasus until the early 8th century. Since Joseph de Guignes in the 18th century, historians have associated the Huns who appeared on the borders of Europe in the 4th century with the Xiongnu who migrated out of the Mongolia region some three hundred years before. Due to the conflict with Han China, the northern branch of the Xiongnu had retreated north-westward; their descendants may have migrated through Eurasia and consequently they may have some degree of cultural and genetic continuity with the Huns. The evidence for continuity between Huns and Xiongnu has not been definitive. A school of modern scholarship instead uses an ethnogenetic, rather than essentialist, approach in explaining the Huns' origin. Contemporary literary sources do not have a clear consensus of the Hun origins. The Huns seem to "suddenly appear", first mentioned during an attack on the Alans, who are generally connected to the River Don (Tanais). Scholarship from the early 20th century literature connected the sudden and apparently devastating Hun appearance as a predatory migration from the more easterly parts of the steppe, i.e. Central Asia. This interpretation has been formulated on sketchy and hypothetical etymological and historical connections. More recent theories view the nomadic confederacies, such as the Huns, as the formation of several different cultural, political and linguistic entities that could dissolve as quickly as they formed, entailing a process of ethnogenesis. A group of "warrior" horse-nomads would conquer and/or be joined by other warrior groups throughout western Eurasia, and in turn extracted tribute over a territory that included other social and ethnic groups, including sedentary agricultural peoples. In steppe society, clans could forge new alliances and subservience by incorporating other clans, creating a new common ancestral lineage descended from an early heroic leader. Thus, one cannot expect to find a clear origin. "All we can say safely," says Walter Pohl, "is that the name Huns, in late antiquity (4th century), described prestigious ruling groups of steppe warriors." Kim rejects "old racial theories or even ethnic affiliations" in favour of the "historical reality of these extensive, multiethnic, polyglot steppe empires". The name Hun was used to refer to groups over wide and often discontiguous geographic regions, referred to by disparate sources (including Indic, Persian, Chinese, Byzantine, Roman). After the Hun era in Europe, Greek and Latin chroniclers continued to use the term "Huns" when referring to tribal groups whom they placed in the Black Sea region.