Axis control of the Danube

Axis control of the Danube was brought about by force of arms, through annexation of Austria, invasion of Yugoslavia and of the Soviet Union and treaties with the Kingdom of Romania and Hungary, but a legal cover was provided through moves that resulted in a new international order on the river beginning in 1940 and ending in 1945. Before World War II, international trade and commerce at the mouths of the Danube, 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.

Source: Wikipedia — Axis control of the Danube (CC BY-SA 4.0)