Socialist Federal Republic of Yugoslavia
Regarding rivers, the Danube was navigable throughout its entire course in Yugoslavia, linking the ports of Belgrade, Novi Sad, and Vukovar with Central Europe and the Black sea. Long stretches of rivers Sava, Drava and Tisza were also navigable.
Nazi rule over the Danube River
In view of these intentions, various proposals to establish permanent German possession of the entire Danubian waterway were formulated by the Nazis. One of these proposed the settlement of all the ethnic Germans in Southeastern Europe along a broad strip of territory on both sides of the river, stretching from Mohács in southern Hungary all the way to the Black Sea. Another suggested turning Belgrade into a fortress-city of the Reich ("Reichsfestung") and the center of a "Reichsgau" (administrative subdivision), to be named Prinz-Eugen. Neither of these ideas was officially endorsed by the wartime German government, however, and the historical record indicates that the Nazis had no clear conception as to what extent Germany should control the Danube or how this control was to be effected. Hitler's own preference, at least for the immediate future, was the retention of a number of satellite states (Hungary, Romania, Bulgaria and others) closely tied to the Nazi regime politically and economically. Serbia at the very least was most likely to remain under some permanent form of German administration.