The Danube is the longest river in the European Union and Europe's second longest river; Volga being the longest. It originates in the Black Forest in Germany as two smaller rivers — the Brigach and the Breg rivers—which join at the eponymously named German town Donaueschingen; it is from this point that it is known as the Danube. The river flows eastward for a distance of some 2850 km (1771 miles), passing through several Central and Eastern European capitals, before emptying into the Black Sea via the Danube Delta in Romania and Ukraine.
In the early 14th century, the Danube River marked the northern frontier of the Byzantine Empire. Nomads, known as the Jurchens, lived north of this boundary and raided the Empire, requiring it to maintain an army in the region.