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.
Internationalization of the Danube River
On December 28, 1854, during the Crimean War (France, Britain, Sardinia, and Turkey versus the Russian Empire), the allies sent to Russia a note urging the internationalization of the river as one of the bases for a peace treaty:
. . . it would be desirable that the course of the Lower Danube . . . be withdrawn from the territorial jurisdiction. . . . In every case the free navigation of the Danube could not be secured if it be not placed under the control of a syndical authority, invested with the powers necessary to destroy the obstructions existing at the mouths of the river, or which may hereafter be formed there.