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 centerline of the river represents 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. The cadastre-based boundary reflects the course of the Danube which existed in the 19th century, before anti-meandering and hydrotechnical engineering works altered its course. The area size of the territory in dispute is reported variously, up to 140 km2 (54 square miles).
The dispute first arose in 1947, but was left unresolved during the existence of the Socialist Federal Republic of Yugoslavia. After the breakup of Yugoslavia, the dispute would become a contentious issue. Particular prominence was given to the dispute at the time of Croatia's accession to the European Union. The dispute remains unresolved, and the line of control mostly corresponds to Serbia's claim.