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 Delta Campaign
After annexing Bessarabia and Northern Bukovina from Romania in the summer of 1940, the Soviet Union gained a border on the Danube. The Soviets created a new flotilla on the Danube, which was formed of ships transferred from the Dnieper Flotilla. The new Danube Flotilla consisted of five monitors (armed with 102 mm and 130 mm guns), twenty-two armored boats, and five transports, supported by an anti-aircraft battalion, fighter and bomber squadrons, a rifle company, a machine gun company, a naval infantry company, and eight shore batteries (two 152 mm, one 130 mm, one 122 mm, one 76 mm, and three 45 mm gun batteries).