Croatia–Serbia border dispute

The border between Croatia and Serbia in the area of the Danube is disputed, an important part of their broader diplomatic relations. While Serbia claims that the thalweg of the Danube valley and the centreline of the river represent the international border between the two countries, Croatia disagrees, claiming that the international border lies along the boundaries of the cadastral municipalities located along the river—departing from the course at several points along a 140-kilometre (87 mi) section.

Source: Wikipedia — Croatia–Serbia border dispute (CC BY-SA 4.0)