Croatia–Serbia border

A line never drawn

Before 1991 Croatia and Serbia were republics of Yugoslavia. In 1945 a boundary commission proposed a provisional line between them, and the Communist Party’s Politburo endorsed it that July. But no detailed delimitation was ever carried out, and no Yugoslav law ever defined the republics’ borders.^[2]

When Yugoslavia broke up, the old internal borders became international ones, under the principle known as uti possidetis. The question was where exactly this one ran.^[2] In the war that followed Croatia’s independence, the eastern Slavonia region along the border stayed outside Zagreb’s control until a UN mission handed it back; Croatia regained full sovereignty there in 1998.^[3][4]

River or map

Croatia claims the outer limits of its cadastral districts, drawn from an Austro-Hungarian survey finished in 1891. Just afterwards, in 1891–94, engineers cut new channels to improve navigation, and the river has meandered since, so the old limits leave Croatian “pockets” on the Danube’s left bank, and smaller Serbian ones on the right. Croatia claims 115 km² of the left bank, including parts of the Serbian municipalities of Apatin, Sombor and Bačka Palanka.^[2][1]

Serbia holds that the 1945 commission made the Danube itself the line, and that for a navigable river the border is the thalweg, the navigable channel, which it says international practice supports. It does not accept cadastral limits that ignore a century of change in the river.^[2] Two small islands near Vukovar have also been disputed, and on the agenda of a joint commission, since 1992.^[1]

“Serbia wants the Danube River to form a natural border, while Croatia wants the border to follow the cadastral principle on the grounds that the Danube has changed course over the years.”^[1]

Talks, and Liberland

An inter-state commission founded in 2002 has met only occasionally, with no real progress; a 2021 analysis suggested the International Court of Justice as a possible way out.^[2] The dispute is expected to matter more as Serbia’s talks on joining the EU advance.^[1]

Because Croatia’s claim leaves some right-bank pockets to Serbia, and Serbia’s claim leaves them to Croatia, a few patches are claimed by neither; each would rather the other had them.^[5] In 2015 a group of Czech libertarians declared one of them, Gornja Siga, the micronation of Liberland.^[1]