River Danube. The Danube has been an important international waterway for centuries, as it remains today. Known to history as one of the long-standing frontiers of the Roman Empire, the river flows through—or forms a part of the borders of—ten countries: Germany, Austria, Slovakia, Hungary, Croatia, Serbia, Bulgaria, Romania, Moldova, and Ukraine; in addition, the drainage basin includes parts of ten more countries: Poland, Switzerland, Czech Republic, Slovenia, Bosnia and Herzegovina, Montenegro, Republic of Macedonia, Moldova, and Albania.