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.
Danube Flotilla (Soviet Union)
The Danube Flotilla was a naval force of the Soviet Navy's Black Sea Fleet during World War II (in Russia, called the Great Patriotic War) and afterwards, existing 1940–1941 and 1944–1960. The Flotilla operated on the Danube River and also, at times, on other rivers connected to the Black Sea.