Danube
The Danube flows through many cities, including four national capitals (shown below in bold), more than any other river in the world. Ordered from the source to the mouth they are:
The Danube is navigable by ocean ships from the Black Sea to Brăila in Romania and by river ships to Kelheim, Bavaria, Germany; smaller craft can navigate further upstream to Ulm, Württemberg, Germany. About 60 of its tributaries are also navigable.
Nazi rule over the Danube River
Before World War II, international trade and commerce at the mouths of the Danube River, as well as many of the physical works needed to keep vessels from running aground, were regulated by an international agency called the European Commission of the Danube, founded in 1856. By the time the Hitlerian invasions began in 1938 the commission was composed of the riparian (river-bordering) states, plus France, Great Britain, and Italy, the chief commercial nations of Europe at that time. Above Brăila, the river was under the control of the International Danube Commission, a body composed only of the riparian states.